Reflective mask blank, reflective mask and methods of producing the mask blank and the mask

ABSTRACT

A reflective mask blank has a substrate ( 11 ) on which a reflective layer ( 12 ) for reflecting exposure light in a short-wavelength region including an extreme ultraviolet region and an absorber layer ( 16 ) for absorbing the exposure light are successively formed. The absorber layer ( 16 ) has an at least two-layer structure including as a lower layer an exposure light absorbing layer ( 14 ) formed by an absorber for the exposure light in the short-wavelength region including the extreme ultraviolet region and as an upper layer a low-reflectivity layer ( 15 ) formed by an absorber for inspection light used in inspection of a mask pattern. The upper layer is made of a material containing tantalum (Ta), boron (B), and nitrogen (N). The content of B is 5 at % to 30 at %. The ratio of Ta and N (Ta:N) falls within a range of 8:1 to 2:7. Alternatively, the reflective mask blank has a substrate on which a multilayer reflective film and an absorber layer are successively formed. In this case, the absorber layer is made of a material containing tantalum (Ta), boron (B), and nitrogen (N). The content of B is 5 at % to 25 at %. The ratio of Ta and N (Ta:N) falls within a range of 8:1 to 2:7.

This is a divisional of application Ser. No. 10/510,916 filed Oct. 12,2004 now U.S. Pat. No. 7,390,596. The entire disclosure(s) of the priorapplication(s), application Ser. No. 10/510,916 is considered part ofthe disclosure of the accompanying divisional application and is herebyincorporated by reference.

TECHNICAL FIELD

This invention relates to a reflective mask and a reflective mask blanksuitably used in lithography using exposure light within ashort-wavelength region, such as extreme ultraviolet light, and tomethods of producing the mask and the mask blank and, in particular, toa reflective mask and the like enabling accurate and quick inspection ofa mask pattern.

BACKGROUND ART

In recent years, following the development of a highly-integratedsemiconductor product, such as a semiconductor memory and a VLSI (VeryLarge Scale Integrated circuit), there arises a demand for a finepattern exceeding a transfer limit in photolithography. In order toenable such a fine pattern to be transferred, proposal has been made oflithography using extreme ultraviolet light (hereinafter abbreviated toEUV light) having a shorter wavelength or the like. It is noted herethat the EUV light means light of a wavelength band within a soft X-rayregion or a vacuum ultraviolet region, specifically, light having awavelength of about 0.2-100 nm.

In the meanwhile, proposal has been made of a reflective mask used as anexposure mask within a short-wavelength region including the EUV lightor X-ray. The reflective mask has a basic structure comprising a Si orquartz substrate, a reflective layer formed on the substrate to reflectthe EUV light or the X-ray, and an absorber pattern formed on thereflective layer to absorb the EUV light or the X-ray. Generally, thereflective layer comprises a multilayer film including thin films of atleast two kinds of substances alternately laminated. In a directioninclined at several degrees (typically, 2 to 5 degrees) from aperpendicular direction to the mask, exposure light is incident to themask. The exposure light is absorbed in an area where the absorberpattern is present. In a remaining area, the exposure light is reflectedby the reflective layer. Therefore, a reflected image corresponding tothe absorber pattern is formed. By reduction projection of the reflectedimage through an appropriate optical system onto a silicon wafer,transfer is carried out.

Japanese Patent Application Publication (JP-A) Nos. H7-333829 andH8-213303 disclose a structure including an intermediate layer formedbetween the reflective layer and the absorber, in addition to theabove-mentioned basic structure of the reflective mask. Thus, theintermediate layer is formed in order to protect the reflective layerduring patterning of the absorber, in particular, during etching so thatthe reflective layer as an underlying layer is not damaged by etching.

Now, referring to FIG. 1, description will be made of a method ofproducing a reflective mask used in the lithography using the EUV light(for example, the EUV light having a wavelength of about 13.4 nm withinthe soft X-ray region). FIG. 1 includes schematic sectional viewssequentially showing a production process of an existing reflectivemask.

A mask blank 101 is prepared by successively depositing, on a substrate11 of quartz or the like, a laminate film 12 as a reflective layer forthe EUV light (hereinafter called an EUV reflective layer), a bufferlayer 13 (corresponding to the above-mentioned intermediate layer)formed on the laminate film for the purpose of protecting the EUVreflective layer in an absorber pattern forming step, an absorber layer14 formed on the buffer layer to absorb the EUV light (hereinaftercalled an EUV absorber layer) (see FIG. 1( a)).

Next, the EUV absorber layer 14 as an absorber for the EUV light isprocessed to form an EUV absorber pattern having a predetermined pattern(see FIG. 1( b)).

Then, inspection is carried out to confirm whether or not the EUVabsorber pattern is formed exactly as designed. For example, it isassumed that, as a result of the pattern inspection, detection is madeof occurrence of a pinhole defect (a defect that the absorber layer isremoved at a position where it should not be etched off, may be called awhite (clear) defect) 21 resulting from adhesion of foreign matters to aresist layer during pattern formation and an underetching defect (aposition where the absorber layer is not sufficiently removed due tounderetching, may be called a black (opaque) defect) 22, as shown inFIG. 1( b). In this event, the pinhole defect 21 is repaired bydepositing a carbon film 23 in a pinhole using focused ion beam (FIB)assisted deposition. The underetching defect 22 is repaired by removinga residual part 22 a using FIB gas assisted etching to obtain a removedpart 25 where the absorber layer 14 is removed. By irradiation energyduring repair, a damaged part 24 (a part 24 a removed by FIB and a part24 b penetrated by FIB ions) is present on a surface of the buffer layer13 (see FIG. 1( c)).

Thereafter, by removing a part of the buffer layer 13 corresponding tothe removed part 25 where the EUV absorber layer 14 is removed, apattern 26 is formed so that the reflective mask for the EUV light isproduced (see FIG. 1( d)).

When the reflective mask is exposed by EUV light 31, the light isabsorbed in an area where the absorber pattern is present. In aremaining area (where the absorber layer 14 and the buffer layer 13 areremoved), the EUV light 31 is reflected by the reflective layer 12 whichis exposed (see FIG. 1( e)). Thus, the reflective mask can be used as amask for the lithography using the EUV light.

As described above, in the above-mentioned mask production process,inspection is carried out, after the pattern is formed on the EUVabsorber layer 14, to confirm whether or not the EUV absorber pattern isformed exactly as designed. The inspection of the mask pattern iscarried out by the use of an inspecting apparatus using light having awavelength of, for example, about 257 nm (generally, deep ultravioletlight having a wavelength of 190-260 nm). Specifically, by irradiatingthe mask with the light of about 257 nm, a pattern of a reflected imageis produced to be subjected to the inspection. The inspection of themask pattern is carried out after completion of the pattern forming step(step in FIG. 1( b)) for the EUV absorber layer 14 on a surface asdescribed above. Based on the result of the inspection, the pattern isrepaired if necessary. Specifically, the inspection is carried out by adifference in reflectivity between the surface of the buffer layer 13exposed after the absorber on the surface is removed by patterning andthe surface of the absorber as the remaining pattern when the light usedin the inspection (hereinafter will be referred to as inspection light)is irradiated onto the mask. Therefore, if the difference inreflectivity for the wavelength of the inspection light between thesurface of the buffer layer and the surface of the absorber layer issmall, a contrast upon the inspection is insufficient so that defectinspection can not accurately be carried out.

Typically, in case of the existing reflective mask, the EUV absorber onthe surface is formed by a tantalum film or a tantalum nitride film andthe buffer layer is formed by a SiO₂ film. For the inspection lighthaving a wavelength of 257 nm, the difference between the reflectivityof the surface of the absorber and the reflectivity on the surface ofthe buffer layer is small so that the contrast upon the inspection isinsufficient. As a result, a pattern defect can not sufficiently bedetected during the inspection of the mask and an accurate defect testcan not be carried out.

On the other hand, by inspection with an electron microscope using anelectron beam, an EUV absorber film is damaged by the electron beamirradiated thereto. Therefore, practical use is difficult.

Proposal is also made of a method of using light of about 13.4 nm as anEUV light wavelength mentioned above to inspect a mask pattern. However,in order to equip the inspection apparatus with an EUV light source, anextremely high facility cost is required. Further, as compared with anexisting inspection apparatus using an ultraviolet wavelength, a patterninspecting step is increased in scale and complicated because astructure of holding a whole of an optical system in vacuum is requiredin order to avoid absorption in atmospheric air. In addition, athroughput is reduced due to a time required for evacuation into vacuum.

Referring to FIG. 1, description will be made of a specific examplewhere the reflective layer 12 is a multilayer reflective film.Specifically, the multilayer reflective film comprising thin films madeof substances different in refractive index and alternately laminated isgenerally used as the reflective layer 12. For example, as themultilayer reflective film for light having a wavelength of about 13 nm,a multilayer film comprising Si and Mo alternately laminated in about 40periods is known.

In the specific example, it is assumed that the buffer layer 13comprises a SiO₂ film or a Cr film and the absorber layer 14 is made ofTa or a Ta alloy.

After the step in FIG. 1( d) (the step of removing a predetermined partof the buffer layer 13 on the reflective layer 12 to form the pattern26), inspection for final confirmation is carried out to confirm whetheror not the absorber pattern is formed in exact conformity with aspecification. The final inspection of the pattern is also carried outby observing the contrast in reflection of the inspection light on thesurface of the mask using deep ultraviolet light as the inspectionlight, in the manner similar to the above-mentioned first inspectionafter completion of the pattern forming step (the step in FIG. 1( b))for the absorber layer 14.

Specifically, in the first inspection, the inspection is carried out byobserving the contrast in reflection of the inspection light between thesurface of the buffer layer 13 exposed in the area where the absorberlayer 14 is removed and the surface of the absorber layer 14 in the areawhere the absorber layer 14 remains. On the other hand, in theinspection for final confirmation, the inspection is carried out by thecontrast in reflection of the inspection light between the surface ofthe multilayer reflective film 12 exposed in the area where the bufferlayer 13 is removed and the surface of the absorber layer 14 in the areawhere the absorber layer 14 remains.

Therefore, if the difference in reflectivity between the surface of thebuffer layer 13 and the surface of the absorber layer 14 for thewavelength of the inspection light is small, the contrast during thefirst inspection is inferior so that the first inspection can notaccurately be carried out. If the difference in reflectivity between thesurface of the multilayer reflective film and the surface of theabsorber layer for the wavelength of the inspection light is small, thecontrast during the final inspection is inferior so that the finalinspection can not accurately be carried out.

For example, in case where the deep ultraviolet light having awavelength of 257 nm is used as the inspection light, the reflectivityof Ta or the Ta alloy used as the absorber layer for the EUV light is asrelatively high as about 35%. On the other hand, the reflectivity of thebuffer layer is about 40% in case of SiO₂ and about 50% in case of Cr.Therefore, the difference in reflectivity is small so that a sufficientcontrast can not be obtained in the pattern inspection. A Mo/Si periodicmultilayer film generally used for the exposure light having awavelength of about 13 nm has a reflectivity of about 60% for farultraviolet light. In this case, it is also difficult to achieve acontrast sufficient to obtain an accurate result in the inspection forfinal confirmation.

On the other hand, it is possible to decrease the reflectivity for theinspection light by roughening the surface of the absorber layer. Inthis case, however, edge roughness after pattern formation is increasedso that the dimensional accuracy of the mask is degraded.

In order to decrease the reflectivity, it is effective to add nitrogen.However, tantalum nitride (TaN) containing Ta with nitrogen addedthereto is a crystalline substance. In particular, if a metal film isused as the buffer layer, a TaN film formed thereon has a granularstructure. In this case also, the edge roughness after pattern formationis increased and the dimensional accuracy of the mask is degraded.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a reflective mask and areflective mask blank which enable mask pattern inspection to beaccurately and quickly carried out and to provide methods of producingthe mask and the mask blank.

As a result of diligent study in order to achieve the above-mentionedobject, the present inventor has found out that, by functionallyseparating an absorber layer on a surface of an existing mask into alayer for absorbing exposure light and a layer having a low reflectivityfor a mask pattern inspection wavelength and by laminating these layers,a sufficient contrast is obtained upon pattern inspection.

Specifically, a mask blank according to this invention is a mask blankcomprising a substrate on which a reflective layer for reflectingexposure light in a short-wavelength region including an EUV region, abuffer layer for protecting the reflective layer during formation of amask pattern, and an absorber layer for absorbing the exposure light aresuccessively formed, wherein the absorber layer has an at leasttwo-layer structure including as a lower layer an exposure lightabsorbing layer comprising an absorber for the exposure light within theshort-wavelength region including the EUV region and as an upper layer alow-reflectivity layer comprising an absorber for inspection light usedin inspection of the mask pattern.

A reflective mask according to this invention is obtained by patterningat least the low-reflectivity layer and the exposure light absorbinglayer in the above-mentioned mask blank.

The reflective mask according to this invention is applicable to a maskfor EUV light. The exposure light has a wavelength within the EUVregion, specifically, within a wavelength region of several nanometersto 100 nanometers.

Specifically, the low-reflectivity layer as an uppermost layer may beformed by a material having a low reflectivity for the wavelength of themask pattern inspection light.

In this invention, the absorber layer has a laminated structure in whichthe absorber layer is functionally separated into the layer forabsorbing the exposure light (exposure light absorbing layer) and thelow-reflectivity layer for the inspection light. With this structure, anexposure light absorbing function as a primary function is not impairedand the reflectivity for the pattern inspection wavelength is remarkablydecreased by the low-reflectivity layer formed on an uppermost surface.Thus, a difference in reflectivity between the surface of thelow-reflectivity layer and the surface of the buffer layer exposed afterthe absorber layer is removed by pattern formation is increased at thepattern inspection wavelength. Therefore, a sufficient contrast uponinspection is achieved and a reflected image pattern of a high contrastis formed. Consequently, it is possible to accurately and quicklyinspect the mask pattern by the use of a mask inspection apparatus usedat present.

By functionally separating the absorber layer into the layer forabsorbing the exposure light (exposure light absorbing layer) and thelow-reflectivity layer for the inspection light, it is possible tooptimize light absorption and light reflection characteristicsindividually for the exposure light and the inspection light and tofurther reduce the thickness. Although the absorber layer has alaminated structure, the thickness is suppressed to a level equivalentto that of an existing single-layer structure. Therefore, it is possibleto suppress blurring at an edge portion of the pattern during exposure.Further, by reducing a processing time for pattern formation, a patterndamage is minimized to achieve an improvement in quality.

Preferably, the exposure light absorber of the lower layer in theabsorber layer is made of at least one substance selected from a lowerlayer substance group including one element selected from an elementgroup including, for example, chromium, manganese, cobalt, copper, zinc,gallium, germanium, molybdenum, palladium, silver, cadmium, tin,antimony, tellurium, iodine, hafnium, tantalum, tungsten, titanium, andgold, a substance containing at least one of nitrogen and oxygen and theabove-mentioned one element selected, an alloy containing one elementselected from the element group, and a substance containing at least oneof nitrogen and oxygen and the above-mentioned alloy.

The above-mentioned alloy containing one element includes an alloy ofthe above-mentioned elements, such as a tantalum germanium alloy (TaGe),an alloy with silicon, such as a tantalum silicon alloy (TaSi) or atantalum germanium silicon alloy (TaGeSi), an alloy with boron, such asa tantalum boron alloy (TaB), a tantalum silicon boron alloy (TaSiB), ora tantalum germanium boron alloy (TaGeB), and so on.

Preferably, the inspection light absorber forming the low-reflectivitylayer as the upper layer of the absorber layer is made of at least onesubstance selected from an upper layer substance group including one ofnitride, oxide, and oxynitride of a substance forming the exposure lightabsorber, one of the nitride, the oxide, and the oxynitride with siliconadded thereto, and oxynitride of silicon.

A method of producing a mask blank according to this invention comprisessteps of forming on a substrate a reflective layer for reflectingexposure light in a short-wavelength region including an EUV region,forming on the reflective layer a buffer layer for protecting thereflective layer during formation of a mask pattern, forming on thebuffer layer an exposure light absorbing layer for the exposure light inthe short-wavelength region including the EUV region, and forming on theexposure light absorbing layer a low-reflectivity layer for inspectionlight used in inspection of the mask pattern. Depending upon a materialof the absorber, the low-reflectivity layer for the inspection lightused in the inspection of the mask pattern may be formed by treating asurface of the exposure light absorbing layer for the exposure light inthe short-wavelength region including the EUV region after the exposurelight absorbing layer is formed on the buffer layer.

In the latter technique, a work can be simplified and a working time canbe shortened.

In the method of producing a mask blank according to this invention, itis preferable to obtain a relationship between a thickness of thelow-reflectivity layer formed on the exposure light absorbing layer forthe exposure light and the reflectivity on the low-reflectivity layerfor the inspection light wavelength and to select the thickness of thelow-reflectivity layer so that the reflectivity on the low-reflectivitylayer for the inspection light wavelength is minimized.

The reflective mask according to this invention is produced bypatterning the low-reflectivity layer as the upper layer and theexposure light absorbing layer as the lower layer which form theabsorber layer of the mask blank. Preferably, after the low-reflectivitylayer and the exposure light absorbing layer are patterned, the bufferlayer is removed in an area where the low-reflectivity layer and theexposure light absorbing layer are removed. By removing the bufferlayer, the reflective mask is improved in reflection characteristic forthe exposure light.

As a result of diligent study in order to achieve the above-mentionedobject, the present inventor has found out that, by selecting a specificmaterial as the material of the absorber layer, a sufficient contrast isobtained in pattern inspection without degrading the dimensionalaccuracy of the mask.

The present inventor has found out that the above-mentioned object isachieved by using as the absorber layer a material containing tantalum,boron, and at least one element selected from oxygen and nitrogen.

Specifically, a reflective mask blank according to this invention is areflective mask blank comprising a substrate on which a multilayerreflective film for reflecting exposure light and an absorber layer forabsorbing the exposure light are successively formed, with a bufferlayer formed between the multilayer reflective film and the absorberlayer to protect the multilayer reflective film during etching forpattern formation on the absorber layer, wherein the absorber layer ismade of a material containing tantalum (Ta), boron (B), and nitrogen(N), the composition of Ta, B, and N being selected so that the contentof B is 5 at % to 25 at % and that the ratio of Ta and N (Ta:N) fallingwithin a range of 8:1 to 2:7.

Alternatively, a reflective mask blank according to this invention is areflective mask blank comprising a substrate on which a multilayerreflective film for reflecting exposure light and an absorber layer forabsorbing the exposure light are successively formed, with a bufferlayer formed between the multilayer reflective film and the absorberlayer to protect the multilayer reflective film during etching forpattern formation on the absorber layer, wherein the absorber layer ismade of a material containing tantalum (Ta), boron (B), and oxygen (O).In this case, the material of the absorber layer may further containnitrogen (N).

Preferably, the material of the absorber layer has an amorphous state.

Preferably, the buffer layer combined with the material of the absorberlayer in this invention is made of a material containing chromium (Cr).

A reflective mask according to this invention is obtained by patterningthe absorber layer of the reflective mask blank.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows schematic sectional views illustrating a production processof an existing reflective mask.

FIG. 2 is a schematic sectional view of a mask blank according to afirst embodiment of this invention.

FIG. 3 is a schematic sectional view of a reflective mask formed byusing the mask blank in FIG. 2.

FIG. 4 is a view for describing this invention, in which a reflectivityR at an inspection wavelength of 190 nm in case where a low-reflectivitylayer is formed to a thickness of 10 nm by the use of materials havingvarious values of a refractive index n and an extinction coefficient kis plotted on axes n and k;

FIG. 5 is a view for describing this invention, in which a reflectivityR at an inspection wavelength of 260 nm in case where a low-reflectivitylayer is formed to a thickness of 10 nm by the use of materials havingvarious values of a refractive index n and an extinction coefficient kis plotted on axes n and k;

FIG. 6 is a view for describing this invention, in which a reflectivityR at an inspection wavelength of 190 nm in case where a low-reflectivitylayer is formed to a thickness of 20 nm by the use of materials havingvarious values of a refractive index n and an extinction coefficient kis plotted on axes n and k;

FIG. 7 is a view for describing this invention, in which a reflectivityR at an inspection wavelength of 260 nm in case where a low-reflectivitylayer is formed to a thickness of 20 nm by the use of materials havingvarious values of a refractive index n and an extinction coefficient kis plotted on axes n and k;

FIG. 8 shows schematic sectional views illustrating a production processof the reflective mask in FIG. 3.

FIG. 9 is a schematic view of a pattern transfer apparatus using thereflective mask in FIG. 3.

FIG. 10 is a view showing values of the reflectivity for lights havingwavelengths from 190 nm to 690 nm in an example 1-1 of this inventionand in the existing reflective mask.

FIG. 11 is a view showing values of the reflectivity for lights havingwavelengths from 190 nm to 690 nm in an example 1-2 of this inventionand in the existing reflective mask.

FIG. 12 is a view showing values of the reflectivity for lights havingwavelengths from 190 nm to 690 nm in an example 1-3 of this inventionand in the existing reflective mask.

FIG. 13 is a view for describing a TaBN/TaBO intermediate region in anexample 1-11 of this invention.

FIG. 14 show schematic sectional views for describing a productionprocess of a reflective mask according to a second embodiment of thisinvention.

FIG. 15 is a schematic view for describing an inspection method for anabsorber pattern of the reflective mask according to the secondembodiment of this invention.

FIG. 16 is a schematic view for describing an inspection method for anabsorber pattern of the reflective mask according to the secondembodiment of this invention.

FIG. 17 is a schematic view of a pattern transfer apparatus for carryingout pattern transfer onto a semiconductor substrate by the use of thereflective mask in FIG. 14.

BEST MODE FOR EMBODYING THE INVENTION First Embodiment

Now, a first embodiment of this invention will be described in detailwith reference to the drawing.

FIG. 2 is a schematic sectional view of a mask blank according to thefirst embodiment of this invention and FIG. 3 is a schematic sectionalview of a reflective mask formed by using the mask blank in FIG. 2.

The mask blank according to the first embodiment of this invention has astructure illustrated in FIG. 2. Specifically, the mask blank 1comprises a substrate 11 on which a reflective layer 12 for reflectingexposure light in a short-wavelength region including an EUV region, abuffer layer 13 for protecting the reflective layer 12 during formationof a mask pattern, and an absorber layer 16 for absorbing the exposurelight are successively formed. In this embodiment, the absorber layer 16has a two-layer structure comprising as a lower layer an exposure lightabsorbing layer 14 formed by an absorber for the exposure light in theshort-wavelength region including the EUV region and as an upper layer alow-reflectivity layer 15 for inspection light used in inspection of themask pattern.

As shown in FIG. 3, a reflective mask 2 in this invention is obtained bypatterning at least the absorber layer 16, i.e., the low-reflectivitylayer 15 and the exposure light absorbing layer 14, in the mask blank 1.

In the reflective mask in this invention, the absorber layer has alaminate structure in which the absorber layer on a the surface of themask is functionally separated into a layer for absorbing the exposurelight and a layer having a low reflectivity for a mask patterninspection wavelength. Thus, a sufficient contrast during inspection ofthe mask pattern is obtained.

The reflective mask in this invention makes it possible to transfer afine pattern beyond a transfer limit by the known photolithography.Therefore, the reflective mask may be used in lithography using light inthe short-wavelength region including the EUV region and may be used asa reflective mask for the EUV light.

Next, description will be made of a structure of each layer.

Generally, a quartz glass or a silicon wafer optically polishedappropriately is used as the substrate 11. The size and the thickness ofthe substrate 11 are appropriately determined depending upon designedvalues of the mask and may have any desired values in this invention.

The exposure light reflective layer 12 is formed by a materialreflecting the exposure light in the short-wavelength region includingthe EUV region. As a matter of course, it is particularly preferable toform the reflective layer by a material having an extremely highreflectivity for the light in the short-wavelength region such as EUVlight because a contrast when used as a reflective mask is enhanced. Forexample, as the reflective layer for the EUV light having a wavelengthof about 12-14 nm in a soft X-ray region, a periodic laminate filmcomprising thin films of silicon (Si) and molybdenum (Mo) alternatelylaminated. Typically, these thin films (having a thickness on the orderof several nanometers) are laminated in 40-50 periods (the number ofpairs of layers) to form a multilayer film. The multilayer film isdeposited, for example, by ion beam sputtering or magnetron sputtering.

As described above, the buffer layer 13 is formed in order to protectthe reflective layer 12 as an underlying layer from being damaged byetching when the mask pattern is formed on the absorber layer 16 for theexposure light on a surface as described above.

Therefore, as a material of the buffer layer 13, selection is made of asubstance which is hardly affected by etching of the absorber layer 16on the surface of the mask, i.e., which is lower in etching rate thanthe absorber layer 16 and is hardly subjected to an etching damage andwhich can be removed later by etching. For example, substances such asCr, Al, Ru, Ta, nitride thereof, SiO₂, Si₃N₄, and Al₂O₃ are preferable.From these substances, an appropriate substance is selected taking intoaccount the material of the absorber layer 16 and the etching technique.The buffer layer 13 must be removable later in the following reason. Byremoving, after the absorber layer 16 is patterned, the buffer layer 13in an area where the absorber layer 16 is removed to expose the surfaceof the reflective layer 12, the reflection characteristic of thereflective mask for the exposure light is improved. Thus, such removalof the buffer layer is desirable. If the above-mentioned substance suchas Cr is selected, the buffer layer 13 is given a function as anexposure light absorbing layer because the substance has an absorptioncharacteristic for the EUV light. Correspondingly, the thickness of theabsorber layer 16 as an overlying layer can be reduced. It is thereforepossible to suppress blurring at an edge portion of the pattern duringexposure. Further, a pattern damage is reduced by shortening aprocessing time for pattern formation. In this case, however, it isessential to remove the buffer layer 13 in the area where the absorberlayer 16 is removed by patterning.

It is desired that the thickness of the buffer layer 13 is small. Thisis because, if the thickness of the buffer layer 13 is large, adifference in height between the surface of the reflective layer 12 andthe surface of the absorber layer 16 is increased as is obvious fromFIG. 3. In this event, there arises a disadvantage that the edge portionof the mask pattern blurs in relation to an optical path of EUV exposurewith an incident angle of about 5 degrees. Further, also in case wherethe buffer layer 13 is later removed by etching, a reduced thickness isdesirable because the processing time can be shortened. Therefore, thethickness of the buffer layer 13 is 100 nm or less, preferably 80 nm orless.

The buffer layer 13 may be deposited by a well-known depositiontechnique, such as magnetron sputtering and ion beam sputtering, like incase of the reflective layer 12.

The buffer layer is provided if necessary. Depending upon the method andthe condition of pattern formation onto the absorber, the absorber layermay be formed directly on the reflective layer.

As already described, the absorber layer 16 has at two-layer structureincluding as the lower layer the exposure light absorbing layer 14comprising the absorber for the exposure light in the short-wavelengthregion including the EUV region and as the upper layer thelow-reflectivity layer 15 for the inspection light used in inspection ofthe mask pattern. In this invention, the absorber layer 16 has alaminated structure in which the absorber layer is functionallyseparated into the exposure light absorbing layer and thelow-reflectivity layer for the inspection light.

The exposure light absorbing layer 14 as the lower layer is made of amaterial absorbing light in the short-wavelength region, such as EUV.Preferably, such exposure light absorber is made of at least onesubstance selected from a lower layer substance group including oneelement selected from an element group including, for example, chromium,manganese, cobalt, copper, zinc, gallium, germanium, molybdenum,palladium, silver, cadmium, tin, antimony, tellurium, iodine, hafnium,tantalum, tungsten, titanium, and gold, a substance containing at leastone of nitrogen and oxygen and the above-mentioned one element selected,an alloy containing one element selected from the element group, and asubstance containing at least one of nitrogen and oxygen and theabove-mentioned alloy.

For example, in case of tantalum, use may be made of a tantalum element(Ta), tantalum nitride (TaN), tantalum oxide (TaO), tantalum siliconalloy (TaSi), tantalum silicon nitride (TaSiN), tantalum boron alloy(TaB), tantalum boron nitride (TaBN), tantalum germanium alloy (TaGe),and tantalum germanium nitride (TaGeN).

As minimum characteristics required for the inspection lightlow-reflectivity layer 15 as the upper layer, it is essential to exhibitlow reflection for the mask pattern inspection wavelength, to allowpattern formation, not to be etched when the buffer layer is etched off(to have an etch selectivity with respect to the buffer layer). Further,to have an EUV light absorbing function is more preferable because thetotal thickness of the absorber layer 16 can be reduced.

The inspection of the mask pattern is generally carried out by the useof deep ultraviolet light having a wavelength on the order of 190-260nm, for example, light having a wavelength of 257 nm mentioned above or193 nm. As a material having a low reflectivity for such inspectionlight wavelength, use may be made of, for example, nitride, oxide, oroxynitride of the substance forming the exposure light absorber, amaterial containing silicon in addition thereto, or silicon oxynitride.

As the material of the low-reflectivity layer, nitride has an effect oflowering the reflectivity at the inspection wavelength and, in case of apolycrystalline film, has an effect of reducing a crystal grain size toimprove smoothness. Oxide has a greater effect of lowering thereflectivity at the inspection wavelength than that of nitride. Silicidehas a less effect of lowering the reflectivity at the inspectionwavelength but has an effect of widening a wavelength region over whichthe reflectivity is lowered. Thus, nitride or oxide provides a curvehaving a minimum value of reflectivity at a specific wavelength portion.If silicon is added to those substances, a low reflectivity is obtainedin a wider wavelength range (see FIGS. 10 and 11 in Example 1-1 andExample 1-2 which will later be described). When the low reflectivity isobtained in a wider wavelength range, it is possible to flexiblyaccommodate the change in inspection wavelength. Further, the change inreflectivity is small even if the minimum value is shifted due to thechange in thickness of the uppermost layer. Therefore, an allowancevalue for deviation from a designed thickness is increased so that aconstraint imposed upon production is relaxed.

Therefore, as the material of the low-reflectivity layer, nitrogen oroxygen must be contained in a compound. As described above, thelow-reflectivity layer preferably comprises at least one substanceselected from an upper layer substance group including nitride, oxide,or oxynitride of a substance forming the exposure light absorber, asubstance containing one of nitride, oxide, and oxynitride and silicon,and silicon oxynitride.

Boride does not much contribute to the reflectivity but is involved inamorphization of a film to contribute to the smoothness of the film.Therefore, by containing boron into the compound, the smoothness of thefilm of the low-reflectivity layer is improved.

Herein, specific examples of the material of the low-reflectivity layerwill be given: oxide, nitride, or oxynitride of a metal used as theexposure light absorbing layer as the lower layer, oxide, nitride, oroxynitride of an alloy of boron and the metal used as the exposure lightabsorbing layer as the lower layer, oxide, nitride, or oxinitride of analloy of silicon and the metal used as the exposure light absorbinglayer as the lower layer, oxide, nitride, or oxynitride of an alloy ofsilicon, boron, and the metal used as the exposure light absorbing layeras the lower layer, and so on. For example, if tantalum is used as themetal of the exposure light absorber, use may be made of tantalum oxide(TaO), tantalum nitride (TaN), tantalum oxynitride (TaNO), tantalumboron oxide (TaBO), tantalum boron nitride (TaBN), tantalum boronoxynitride (TaBNO), tantalum silicon oxide (TaSiO), tantalum siliconnitride (TaSiN), tantalum silicon oxynitride (TaSION), tantalum siliconboron oxide (TaSiBO), tantalum silicon boron nitride (TaSiBN), tantalumsilicon boron oxynitride (TaSiBNO), tantalum germanium nitride (TaGeN),tantalum germanium oxide (TaGeO), tantalum germanium oxynitride(TaGeNO), tantalum germanium silicon nitride (TaGeSiN), tantalumgermanium silicon oxide (TaGeSiO), tantalum germanium silicon oxynitride(TaGeSiNO), and so on.

When the thickness of the low-reflectivity layer is changed, a positionof the minimum value of the reflectivity curve is shifted. For example,in case of a tantalum-based material such as TaO or TaSiON or amolybdenum-based material, the position is shifted towards a longerwavelength if the thickness is increased. Therefore, by changing thethickness of the low-reflectivity layer, the reflectivity at a specificwavelength is changed. Accordingly, it is possible to control thereflectivity by adjusting the thickness to some extent so that thereflectivity at the inspection wavelength is minimum. However, since anexcessively large thickness of the low-reflectivity layer is notpreferable as will later be described, the thickness is adjusted withina range between 5 and 30 nm, preferably between 10 and 20 nm. Bychanging the composition ratio of the material of the low-reflectivitylayer, for example, the composition ratio between a metal and oxygen ornitrogen, the reflectivity is changed. Generally, when the compositionratio of oxygen or nitrogen is increased, the reflectivity is loweredand the absorptivity for the EUV light tends to be lowered.

Comparing nitride and oxide, there is a tendency that oxide has agreater effect of lowering the reflectivity as described above.Therefore, as the material of the low-reflectivity layer, a materialcontaining a metal, oxygen, and silicon (for example, a materialcontaining a metal, oxygen, and silicon as main components, a materialcontaining a metal, silicon, oxygen, and nitrogen as main components,and so on) is most preferable in view of a low reflectivity and a widewavelength range over which the reflectivity is lowered. Herein, it isfurther preferable to use a metal element used as the exposure lightabsorber because the low-reflectivity layer has an EUV light absorptionfunction.

Although the wavelength region over which the reflectivity is lowered isslightly narrower, oxide without containing silicon provides a lowreflectivity in a specific wavelength region. Depending upon thematerial, mere inclusion of nitrogen may not provide a sufficientreduction in reflectivity but, as compared with a metal element, nitridethereof is lowered in reflectivity. Further, addition of nitrogenprovides an effect of improving the smoothness of the film as describedabove. If the smoothness of the film is poor, edge roughness of thepattern is increased and the dimensional accuracy of the mask isdegraded. Therefore, the film is desirably as smooth as possible.

As the material of the low-reflectivity layer, use may be made of amaterial which does not contain a metal but comprises, for example,silicon, nitrogen, and oxygen (silicon oxynitride). In this case,however, the effect of absorbing the EUV light in the low-reflectivitylayer is small.

For example, if the low-reflectivity layer comprises a materialcontaining a metal, Si, N, and O, a composition ratio for obtaining alow reflectivity at the deep ultraviolet light of about 190-260 nm asthe inspection wavelength is preferably 20-25 at % metal such astantalum, molybdenum, or chromium, 17-23% Si, 15-20% N, and the balanceO. The ratio of Si and O is preferably 1:1.5 to 1:2.

In order to obtain a smooth surface of the absorbing layer, thelow-reflectivity layer preferably comprises a film having an amorphousstructure. For example, in case of Ta, amorphization is achieved byinclusion of an appropriate amount of B.

Addition of Si or Ge to Ta also provides the film having the amorphousstructure and is therefore preferable.

For example, in case where the low-reflectivity layer comprises tantalumboron nitride (TaBN), the content of N is preferably 30-70 at %, morepreferably 40-60 at % as the composition ratio for obtaining the lowreflectivity at the above-mentioned inspection wavelength. If thecontent of N is small, a sufficient low-reflection characteristic cannot be obtained. On the contrary, if the content is excessively large,acid resistance is degraded. Further, if both of the low-reflectivitylayer and the absorbing layer under the low-reflectivity layer comprisethe tantalum boron nitride, the content of N in the low-reflectivitylayer is 30-70 at %, preferably 40-60 at % while the content of N in theexposure light absorbing layer is 0-25 at %, preferably 5-20 at %. Asmaller content of N in the absorbing layer is not preferable in view ofthe surface roughness. On the contrary, a greater content lowers theabsorption coefficient for the EUV light.

In case of the TaBN film, the content of B is 5-30 at %, preferably 5-25at %. The ratio of Ta and N is preferably 8:1 to 2:7.

In case where the low-reflectivity layer comprises tantalum boron oxide(TaBO), the content of O is 30-70 at %, preferably 40-60 at %. If thecontent of O is small, the low-reflection characteristic can not beobtained. On the contrary, if the content is great, insulation isincreased so that charge-up occurs by electron beam irradiation. In casewhere the low-reflectivity layer comprises tantalum boron oxynitride(TaBNO), it is preferable that the content of N is 5-70 at % and thecontent of O is 5-70 at %.

In case of the TaBO film, it is preferable that the content of B is 5-25at % and the ratio of Ta and O falls within a range of 7:2 to 1:2. Incase of the TaBNO film, it is preferable that the content of B is 5-25at % and the ratio of Ta and N+O, i.e., Ta:(N+O) falls within a range of7:2 to 2:7.

In each of the above-mentioned substances containing boron, the ratio ofB is preferably 5-30%, more preferably 5-25%, in order to form theamorphous structure.

In the meanwhile, description will be made of a combination of materialsof the exposure light absorbing layer 14 as the lower layer and thelow-reflectivity layer 15 as the upper layer. Preferably, a metal usedin the exposure light absorbing layer 14 is contained in thelow-reflectivity layer 15. For example, if a material containingtantalum is used as the exposure light absorbing layer, thelow-reflectivity layer is also made of a material containing tantalum.Specifically, as the exposure light absorbing layer, a materialcontaining tantalum, for example, one substance selected from a Taelement, TaN, TaB, TaBN, TaBO, and TaBNO may be used. As thelow-reflectivity layer, a material containing tantalum and nitrogen oroxygen, for example, one substance selected from TaO, TaBO, TaBNO, TaNO,TaSiO, and TaSION may be used. Thus, the low-reflectivity layer using ametal same as that of the exposure light absorbing layer providesvarious advantages. Specifically, since a metal having an EUV lightabsorption function is contained, the low-reflectivity layer has the EUVlight absorption function to some extent. Since materials high in etchselectivity are selected as the buffer layer and the exposure lightabsorbing layer, the etch selectivity is basically large also betweenthe buffer layer and the low-reflectivity layer. The exposure lightabsorbing layer and the low-reflectivity layer can be deposited in asame deposition chamber. The exposure light absorbing layer and thelow-reflectivity layer can be patterned under the same etchingcondition.

It is more preferable to use a film having an amorphous structure or afine crystalline structure as the material of the lower layer becausethe film excellent in smoothness is obtained.

As to the reflectivity, by obtaining the relationship between thecomposition of the material of the low-reflectivity layer and thereflectivity on the surface of the absorbing layer and the relationshipbetween the thickness and the reflectivity, it is possible to determinethe composition and the thickness with which the low reflectivity isobtained at the inspection wavelength used.

In the reflective mask and the reflective mask blank according to thisinvention, the surface roughness of the surface of the absorber layer ispreferably 0.5 nmRms or less, more preferably 0.4 nmRms or less, furtherpreferably 0.3 nmRms or less. If the surface roughness of the surface ofthe absorber layer is large, the edge roughness of the absorber patternis increased and the dimensional accuracy of the pattern is degraded. Asthe pattern is finer, the influence of the edge roughness is remarkableand, therefore, the surface of the absorber layer is required to besmooth.

In order to reduce the surface roughness at the surface of the absorber,it is effective that the upper layer of the absorber layer (i.e., thelow-reflectivity layer) comprises a film having an amorphous structure.It is further preferable that the lower layer of the absorber layer alsocomprises a film having an amorphous structure or a fine crystallinestructure and excellent in smoothness. In case where the buffer layer isprovided, it is necessary to use a smooth film as the buffer layer.

Next, description will be made of a combination of the materials of theexposure light absorbing layer 14, the low-reflectivity layer 15, andthe buffer layer 13. In this invention, it is preferable that each ofthe exposure light absorbing layer 14 and the low-reflectivity layer 15is made of a material containing tantalum and the buffer layer 13 ismade of a material containing chromium. By the use of the chromium-basedmaterial as the buffer layer, various advantages are achieved.Specifically, the buffer layer is given the EUV light absorptionfunction as described above. Since the reflectivity for the inspectionlight in the deep ultraviolet region is about 40%, it is easy to designthe reflectivity so that the reflectivity for the inspection wavelengthis lowered successively from the surface of the multilayer reflectivefilm, the surface of the buffer layer, and the surface of the absorberlayer. The etch selectivity with the absorber layer containing tantalumis large. Further, when the buffer layer is removed, almost no damage isgiven to the multilayer reflective film.

As the material containing chromium used as the buffer layer, use ispreferably made of, in addition to a Cr element, a material containingCr and at least one element selected from N, O, and C. For example, usemay be made of chromium nitride (CrN), chromium oxide (CrO), chromiumcarbide (CrC), chromium oxynitride (CrNO), and chromium carboxynitride(CrCNO).

For example, in case of chromium nitride (CrN), a preferable compositionratio of chromium and nitride is given by Cr_(1-X)N_(X) where0.05≦X≦0.5, more preferably 0.05≦X≦0.2. X smaller than 0.05 is notpreferable in view of the acid resistance, the film stress, and thesurface roughness. If X is greater than 0.5, the reflectivity for theinspection light is excessively lowered so that the contrast with thesurface of the absorber layer is not sufficiently large. Further, asmall amount of, for example, about 5% of oxygen, carbon, or the likemay be added to chromium nitride. A CrN film having a fine crystallinestructure is preferable because of an excellent smoothness.

Preferably, the total thickness of the absorber layer 16 comprising theexposure light absorbing layer 14 as the lower layer and the inspectionlight low-reflectivity layer 15 as the upper layer is small. This isbecause the etching process time upon patterning the absorber layer 16is proportional to the thickness. In the etching process, the surface ofthe resist pattern is damaged in correspondence to the etching processtime proportional to the thickness of the absorber layer 16. This bringsabout easy occurrence of nonuniform etching distribution within theplate resulting in an increase of mask pattern defects due to anincrease in frequency of occurrence of white defects and black defects.Further, there arises serious problems such as reduction in massproducibility because a long time is required to repair those defects,and a resultant increase in cost. Further, if the total thickness of theabsorber layer 16 is large, the difference in height between the surfaceof the reflective layer 12 and the surface of the absorber layer 16 isincreased, like in the above-mentioned case where the thickness of thebuffer layer 13 is large. In this event, there arises a disadvantagethat an edge portion of the mask pattern blurs during exposure.

Thus, the total thickness of the absorber layer 16 is 100 nm or less,preferably 80 nm or less, more preferably 60 nm or less. However, if thethickness of the absorber layer 16 is excessively small, the exposurelight absorption characteristic is degraded. Therefore, the thickness ispreferably 35 nm or more at minimum.

In the absorber layer 16, the thickness of the low-reflectivity layer 15as the upper layer is desirably smaller than the thickness of theexposure light absorbing layer 14 as the lower layer. If the thicknessof the low-reflectivity layer 15 as the upper layer is excessivelylarge, the EUV light absorption characteristic of the absorber layer 16as a whole may be degraded. Therefore, the thickness of thelow-reflectivity layer 15 as the upper layer is preferably about 5-30 nmwhile the thickness of the exposure light absorbing layer 14 as thelower layer is preferably about 30-60 nm. As described above, theabsorber layer 16 has a laminated structure but can be suppressed to thethickness substantially equivalent to that of the existing single layerstructure. Further, by providing the buffer layer 13 with the functionas the exposure light absorbing layer, it is possible to correspondinglyreduce the thickness of the exposure light absorbing layer 14 as theupper layer even if the absorption characteristic thereof is degraded.

A preferable range of the total thickness of the buffer layer 13 and theabsorber layer 16 is 60 nm to 130 nm. Although depending upon thematerial, if the total thickness is less than 60 nm, a sufficient EUVlight absorption characteristic may not be obtained. If the totalthickness is greater than 130 nm, the problem of shadow of the patternitself is increased.

Each of the exposure light absorbing layer 14 and the low-reflectivitylayer 15 may be deposited by the use of a known deposition technique,such as magnetron sputtering, ion beam sputtering, CVD, and vapordeposition, like the reflective layer 12 and the buffer layer 13described above.

In the meanwhile, the reflectivity for the wavelength of the patterninspection light is preferably designed so that the reflectivity islowered in the order of the surface of the exposure light reflectivelayer, the surface of the buffer layer, and the surface of thelow-reflectivity layer. The reason is as follows. In each of theinspection between the surface of the buffer layer and the surface ofthe low-reflectivity layer after pattern formation and the inspectionbetween the surface of the exposure light reflective layer and thelow-reflectivity layer after removal of the buffer layer, a portionwhere the pattern is present is dark without reversal of patterncontrast. Therefore, setting of the inspection apparatus need not bechanged and the result is easily recognized. In case of the Mo/Simultilayer film used as the exposure light reflective layer, thereflectivity is as high as about 60%. Therefore, in order to assure asufficient contrast with the respective layers, it is advantageous tolower the reflectivity of the other layers.

Next, description will be made of a relationship between the values ofthe refractive index n and the extinction coefficient k of the materialof the low-reflectivity layer 15 and the reflectivity for the inspectionwavelength.

Referring to FIGS. 4, 5, 6, and 7, chromium nitride is used as thebuffer layer (50 nm), tantalum boron nitride (TaBN) (the content of Nbeing about 18%) is deposited to 50 nm as the exposure light absorbinglayer, and each of materials having different values of the refractiveindex n and the extinction coefficient k is deposited thereon to athickness of 10 nm or 20 nm as the low-reflectivity layer. Thereflectivity R for each of the inspection wavelengths of 190 nm and 260nm is plotted on axes n and k. From the result, it is understood thatthe low reflectivity is obtained by the use of a material satisfying nand k within specific ranges.

Specifically, the relationship among the inspection wavelength, thethickness, and preferable ranges of n and k is as follows.

(1) Comparing the cases where the thickness is 10 nm and 20 nm, thereflectivity is 10% or less in either case if the extinction coefficientk is about 0.7 or less. If the reflectivity is allowed to be 20% orless, k is 1.2 or less. In this event, a preferable range of therefractive index n is slightly different between the cases where thethickness is 10 nm and 20 nm. In case of the thickness of 20 nm, thereflectivity R is 10% or less when n is about 1.5 to 2.5. If thereflectivity is allowed to be 20% or less, n is about 1 to 3. In case ofthe thickness of 10 nm, the reflectivity R is 10% or less when n is 2.0to 3.5. If the reflectivity is 20% or less, n is about 1.5 to 4.0.

(2) Comparing the cases where the inspection wavelength is 190 nm and260 nm, there is no considerable difference. In case of 260 nm, apreferable range of n tends to be slightly shifted towards a greatervalue.

(3) Considering the above in total, it is understood that, in case wherethe thickness is 10 to 20 nm, the reflectivity of 10% or less isobtained within the deep ultraviolet region by selecting a materialhaving the extinction coefficient k of 0.7 or less and the refractiveindex n of 1.5 to 3.5.

The absorber layer 16 may have a so-called laminated structure such as atwo-layer structure in this embodiment or a structure in which nitrogenor oxygen has a predetermined distribution through the absorber layer 16from the side adjacent to the buffer layer 13 towards the surface of theabsorber layer. In this case, by increasing the amount of nitrogen oroxygen towards the surface of the absorber layer, the reflectivity onthe surface of the absorber layer 16 for the inspection wavelength canbe decreased. The distribution of composition of nitrogen or oxygen in athickness direction of the absorber layer may be continuously changedlinearly or in a curved line or may be changed stepwise. Thedistribution of composition of nitrogen or oxygen in the thicknessdirection of the absorber layer can be achieved by controlling theamount of each element which is added during deposition. For example, incase of the TaBN film, sputtering is carried out by the use of a targetcontaining Ta and B. During the sputtering, deposition is carried outwhile changing the amount of a nitrogen gas added. In this manner,continuous or stepwise distribution of composition of nitrogen can beobtained in the thickness direction of the absorber layer 16.

Further, the reflective mask blank and the reflective mask of thisinvention may have an intermediate region formed between the lower layerand the upper layer of the absorber layer and continuously varied incomposition from the composition of the lower layer towards thecomposition of the upper layer.

The intermediate region is a transition region in which the elementcontained in the lower layer and the element contained in the upperlayer are mixed.

By providing the intermediate region, a pattern having a smoothsectional structure is obtained without a boundary formed between theupper and the lower layers when the absorber layer is patterned.

It is preferable that the metal elements contained in the upper and thelower layers are same because the absorber layer can be continuouslypatterned. In addition, there is another advantage that the adhesivenessof the upper and the lower layers is improved.

The intermediate region must have a thickness of about 2 to 15 nm.

Next referring to FIG. 8, description will be made of a method ofproducing a reflective mask according to this invention. FIG. 8 showsschematic sectional view illustrating a production process of thereflective mask according to this invention.

FIG. 8( a) shows the structure of the mask blank 1. The structure hasalready been described. The mask blank 1 is formed by successivelydepositing, on the substrate 11, the exposure light reflective layer 12,the buffer layer 13, the exposure light absorbing layer 14, and theinspection light low-reflectivity layer 15 in this order.

Herein, use may be made of a method of at first depositing the exposurelight absorbing layer 14 on the buffer layer 13 and then depositing theinspection light low-reflectivity layer 15 thereon. Depending upon thematerial of the low-reflectivity layer, for example, in case where thelow-reflectivity layer 15 as the upper layer is made of oxide of a samemetal as that used in the exposure light absorbing layer 14 as the lowerlayer, it is possible to form the inspection light low-reflectivitylayer 15 at an uppermost surface by forming the exposure light absorbinglayer 14 on the buffer layer 13 and thereafter treating the surface ofthe exposure light absorbing layer 14 by oxidization using a process gascontaining oxygen or oxidization using an acid solution. With the lattermethod, it is possible to reduce a time required for changing adeposition condition, to reduce the number of kinds of materials, and toreduce the number of deposition chambers. Thus, the work is simplifiedand the working time is shortened.

Preferably, the low-reflectivity layer as the upper layer and the lowerlayer are continuously formed in the same deposition chamber. In thismanner, it is possible to prevent adsorption of impurities and foreignmatters to the surface of the lower layer and deterioration (oxidation)of the surface from occurring between the lower and the upper layers andto obtain an excellent interface between the lower and the upper layers.

Upon occurrence of adsorption of impurities or deterioration at theinterface between the upper and the lower layers, a stress of theabsorber layer may be changed and optical characteristics, such as aninspection light reflectivity, may be affected. Therefore, parameters atthe interface must be taken into consideration and designedcharacteristics can not be obtained. Thus, a reproducibility or acontrollability is degraded.

On the other hand, if the lower and the upper layers are continuouslyformed in the same deposition chamber, the substrate is not taken outfrom the deposition chamber and is not left unprocessed. Accordingly, anexcellent interface is obtained without adsorption of impurities to theinterface and deterioration. It is therefore possible to form theabsorber layer with excellent reproducibility and excellentcontrollability. There is another advantage that the deposition processis not complicated.

Continuous formation of the upper and the lower layers in the samedeposition chamber is particularly effective in case where the upper andthe lower layers contain metal elements and these metal elements aresame. This is because deposition can continuously be carried out byusing a common source of the metal element and changing a gas suppliedduring deposition.

For example, in case where reactive sputtering is used, continuousdeposition is easily carried out by using a target containing a metalelement common to the upper and the lower layers and changing thecontent of a gas (nitrogen, oxygen, or the like) supplied.

For example, in case where a material containing Ta is used as the upperand the lower layers, a target containing Ta is used in common. Thecontent and the species of a gas (oxygen, nitrogen, or the like)introduced for reduction in reflectivity are changed between theformation of the lower layer and the formation of the upper layer.

By continuous deposition in the same deposition chamber, it is easy tointentionally introduce, between the upper and the lower layers, theabove-mentioned intermediate region where the composition iscontinuously changed.

Specifically, the deposition condition is continuously changed from thedeposition condition of the lower layer to the deposition condition ofthe upper layer. If the same metal element is contained in the lower andthe upper layers in common, a metal element source such as a target isused in common and the flow rate of an introduced gas such as oxygen ornitrogen is changed. Between the formation of the lower layer and theformation of the upper layer, the flow rate of the gas used in theformation of the lower layer is reduced or the flow is stopped while theintroduced amount of the gas used in the formation of the upper layer isincreased or introduction is started with the gas flow rate continuouslychanged. Thus, by carrying out deposition in the state where the gasesused in the formation of the both layers are coexistent, theintermediate region is easily formed.

Next, by processing the absorber layer 16 comprising the exposure lightabsorbing layer 14 as the EUV light absorber and the inspection lightlow-reflectivity layer 15, an absorber pattern having a predeterminedpattern is formed (patterning step, see FIG. 8( b)). Generally, a resistpattern having a predetermined pattern is formed on the surface of theabsorber layer 16 by an electron beam writing process. Next, theabsorber layer is subjected to an etching process. The etching processmay be dry etching or wet etching. Depending upon the material, anappropriate method and an appropriate condition are selected. Finally,the residual resist pattern is removed.

Then, inspection is carried out to confirm whether or not the absorberpattern is formed exactly as designed. As a result of the patterninspection, if a pinhole defect (may also be called a white defect) 21resulting from adhesion of foreign matters to the resist layer duringpattern formation and an underetching defect (may also be called a blackdefect) 22 are present, necessary repair is carried out. The pinholedefect 21 is repaired by depositing a carbon film 23 in a pinhole byfocused ion beam (FIB) assisted deposition. The underetching defect 22is repaired by removing a residual part 22 a by FIB gas assisted etchingto obtain a removed part 25 where the absorber layer 16 having atwo-layer structure is removed. By energy of ion irradiation, a damagedpart 24 (a part 24 a removed by FIB and a part 24 b penetrated by FIBions) is present on the surface of the buffer layer 13 (see FIG. 8( c)).

Next, the buffer layer 13 is removed, for example, by dry etching in anarea corresponding to the removed part 25 where the absorber layer 16 isremoved (buffer layer removing step). At this time, it is important todetermine an etching condition so that etching proceeds in the bufferlayer 13 alone without damaging other layers. By forming a pattern 26 onthe exposure light reflective layer 12 in the above-mentioned manner, areflective mask 2 is produced (see FIG. 8( d)).

The reflective mask 2 produced as mentioned above is exposed to EUVlight 31. The EUV light 31 is absorbed in an area where the absorberpattern is present on the surface of the mask and reflected by thereflective layer 12 exposed in a remaining area where the absorber layer16 and the buffer layer 13 are removed (see FIG. 8( a)). Thus, the maskcan be used as a mask for the lithography using the EUV light.

As mentioned above, in the reflective mask in this invention, theabsorber layer which is a single layer in the existing mask has alaminated structure in which the absorber layer is functionallyseparated into the exposure light absorbing layer 14 as the lower layerand the inspection light low-reflectivity layer 15 as the upper layer.With this structure, a sufficient exposure light absorbing function isassured and the reflectivity on the surface of the inspection lightlow-reflectivity layer 15 as the upper layer formed on an uppermostsurface is remarkably lowered at the wavelength of the patterninspection light. Thus, a difference in reflectivity at the wavelengthof the pattern inspection light between the surface of the inspectionlight low-reflectivity layer 15 and the surface of the buffer layer 13exposed after removing the absorber layer 16 by formation of the maskpattern (see FIG. 8( b)) is increased so that a sufficient contrast uponinspection is obtained. Accordingly, a reflected image pattern having ahigh contrast is obtained. Therefore, by the use of an existing maskinspection apparatus using light having a wavelength within a deepultraviolet region, for example, a wavelength of 257 nm, it is possibleto carry out accurate and quick inspection of the mask pattern, whichhas been difficult so far.

The contrast will further be described. For example, the ratio of thereflectivity of the surface of the inspection light low-reflectivitylayer 15 and the reflectivity of the surface of the buffer layer 13 isgenerally usable as an index of magnitude of the contrast. Also, thefollowing definition formula is known. The value of the definitionformula may be used as the index of magnitude of the contrast.

Specifically, in case where R₁ and R₂ represent values of reflectivityat specific wavelengths, respectively, and R₂ is greater than R₁,contrast (%)={(R ₂ −R ₁)/(R ₂ +R ₁)}×100.

A sufficient contrast must be obtained in the pattern inspection. As arough standard, the ratio of reflectivity is preferably 1:3 or less,more preferably 1:4 or less, further preferably 1:10 or less. Thecontrast value given by the above definition formula is preferably 40%or more, 50% or more, more preferably 60% or more, further preferably80% or more. The contrast value mentioned herein is a contrast betweenthe absorber layer and the reflective layer or a contrast between theabsorber layer and the buffer layer. The reflectivity of thelow-reflectivity layer 15 is preferably 20% or less, more preferably 10%or less, further preferably 5% or less.

EXAMPLES

Now, this invention will be described more in detail in conjunction withspecific examples. For convenience of description, the referencenumerals mentioned in FIGS. 2, 3, and 8 are used as appropriate.

Example 1-1

The respective layers were deposited on the substrate 11 to produce amask blank. Herein, a low-expansion SiO₂—TiO₂ glass substrate having anouter dimension of 6 inch square and a thickness of 6.3 mm was used asthe substrate 11. The glass substrate was subjected to mechanicalpolishing to have a smooth surface of 0.12 nmRms (Rms: root mean squareroughness) and a flatness of 100 nm or less.

At first, on the substrate 11, a Mo/Si laminate film of molybdenum (Mo)and silicon (Si) was deposited by DC magnetron sputtering as the EUVlight reflective layer 12. At first, using a Si target, a Si film wasdeposited to 4.2 nm under an Ar gas pressure of 0.1 Pa. Then, using a Motarget, a Mo film was deposited to 2.8 nm under an Ar gas pressure of0.1 Pa. Defining the above-mentioned deposition as one period, 40periods of deposition was carried out. Finally, a Si film was depositedto 7 nm. The total thickness was 287 nm. The multilayer reflective filmthus obtained had a reflectivity of 60% with respect to light having awavelength of 257 nm.

On the multilayer reflective film, a SiO₂ thin film was deposited to athickness of 50 nm as the buffer layer 13. Specifically, using a Sitarget, deposition was carried out by DC magnetron sputtering using amixed gas of argon (Ar) and oxygen (O₂). The SiO₂ buffer layer had asurface roughness of 0.4 nmRms.

On the buffer layer, a tantalum nitride (TaN) thin film was deposited toa thickness of 50 nm as the exposure light absorbing layer (comprisingan EUV light absorber) 14. Specifically, using a Ta target, depositionwas carried out by DC magnetron sputtering using a mixed gas of argonand nitrogen (N₂). The film had a composition of Ta₆₁N₃₉.

Finally, a TaSiON thin film was deposited to a thickness of 20 nm as thelow-reflectivity layer 15 for the inspection light having a wavelengthof 257 nm. Specifically, using a TaSi alloy target, deposition wascarried out by DC magnetron reactive sputtering using a mixed gas ofargon, oxygen, and nitrogen. The film had a composition ofTa₂₁Si₁₇O₄₇N₁₅. The TaSiON film had a refractive index of 2.09 and anextinction coefficient of 0.24 for light having a wavelength of 260 nmand a refractive index of 2.00 and an extinction coefficient of 0.59 forlight having a wavelength of 190 nm. The TaSiON film had an amorphousstructure. The surface of the TaSiON film had a surface roughness of 0.4nmRms.

Next, using the mask blank produced as mentioned above, a predeterminedmask pattern was formed thereon. Herein, an EUV mask having a 16Gbit-DRAM pattern with 0.07 μm design rule was produced. The maskpattern was formed in the following manner. At first, an electron beamresist material was uniformly applied on a surface of the mask blank bya spinner or the like. After pre-baking, electron beam writing anddevelopment were carried out to form a resist pattern. Next, dry etchingusing a chlorine gas was carried out. After completion of the etching,the resist pattern was removed. Thus, the mask pattern was formed on theexposure light absorbing layer 14 and the low-reflectivity layer 15above the buffer layer 13.

The mask pattern formed as mentioned above was inspected by a maskinspecting apparatus using light having a wavelength of 257 nm. As aresult, a pinhole defect (white defect) and an underetching defect(black defect) were confirmed.

Then, those pattern defects were repaired with reference to theinspection result. Specifically, for the above-mentioned white defect, acarbon film was deposited in a pinhole by focused ion beam (FIB)assisted deposition. For the black defect, a residual part was removedby FIB gas assisted etching. By irradiation energy, a damaged partchanged in optical characteristics due to change in film structure waspresent on the surface of the buffer layer 13 (see (b) and (c) in FIG. 8mentioned above).

Next, a part of the buffer layer 13 exposed in an area without thepatterns of the exposure light absorbing layer 14 and thelow-reflectivity layer 15 was removed by etching (see (d) in FIG. 8mentioned above). At this time, the SiO₂ buffer layer alone wasdry-etched using a fluorine-based gas so that the absorber pattern wasnot damaged but served as an etching mask. Thus, the reflective mask inthis example was produced.

When the EUV light is irradiated onto the mask, the EUV light isreflected by a pattern portion on the surface of the reflective layer12. Thus, a function as the reflective mask is achieved.

For the purpose of comparison, by the existing process illustrated inFIG. 1, preparation was made of a sample comprising an exposure lightabsorbing layer (EUV light absorbing layer) as a single layer withoutthe low-reflectivity layer 15 as the uppermost layer in this example.The exposure light absorbing layer (EUV light absorbing layer) as asingle layer had a thickness of 70 nm equal to the total thickness ofthe two layers including the exposure light absorbing layer (EUV lightabsorbing layer) and the inspection light low-reflectivity layer in thisexample.

FIG. 10 shows the values of reflectivity on the surface of the absorberpattern of the mask with respect to the light having a wavelength withina range from 190 nm to 690 nm. In the figure, the reflectivity on thesurface of the two-layer absorber layer of the mask in this example islabeled TWO LAYER while the reflectivity on the surface of thesingle-layer exposure light absorbing layer (EUV light absorbing layer)of the existing mask is labeled SINGLE LAYER. In the figure, MLrepresents the EUV light reflective layer. It is understood that, in themask of this example, a low-reflectivity wavelength region is relativelywide.

From the above-mentioned result, in case where the pattern inspectionlight had a wavelength of 257 nm, the reflectivity on the surface of thelow-reflectivity layer of the mask in this example was 5.2% at theabove-mentioned wavelength and the reflectivity of the buffer layer(SiO₂) was 42.1% at the above-mentioned wavelength. Therefore, acontrast between the surface of the low-reflectivity layer and thesurface of the buffer layer at the above-mentioned wavelength was 1:8.1in terms of the ratio of reflectivity. The contrast value given by theabove-mentioned definition formula was 78%. The ratio of reflectivitybetween the surface of the low-reflectivity film and the surface of themultilayer film was 1:11.5 and the contrast value was 84%.

On the other hand, the reflectivity on the surface of the absorbinglayer of the existing mask was 43.4% at the above-mentioned wavelength.The contrast between the surface of the absorbing layer and the surfaceof the buffer layer at the above-mentioned wavelength was 1:0.97 interms of the ratio of reflectivity. The contrast value was 1.5%. Theratio of reflectivity between the low-reflectivity film and themultilayer film was 1:1.4. The contrast value was as low as 16%.

In the mask in this example, the reflectivity for the EUV light having awavelength of 13.4 nm was 0.6% and 62.4% on the surface of thelow-reflectivity layer as the upper layer of the absorber layer 16 andon the surface of the EUV light reflective layer, respectively. Thecontrast between the surface of the absorber layer 16 and the surface ofthe reflective layer for the EUV light was 1:104 in terms of the ratioof reflectivity. The contrast value was 98%. Similarly, in the existingmask, the contrast between the surface of the single-layer absorbinglayer and the surface of the reflective layer with respect to the EUVlight was 1:105 and the contrast value was 98%.

Next, description will be made of a method of transferring a pattern bythe EUV light onto a semiconductor substrate (silicon wafer) with aresist by the use of the reflective mask in this example. FIG. 9 shows ageneral structure of a pattern transfer apparatus. The pattern transferapparatus 50 generally comprises a laser plasma X-ray source 32, thereflective mask 2, and a reducing optical system 33. The reducingoptical system 33 comprises an X-ray reflection mirror. The patternreflected by the reflective mask 2 is generally reduced to about ¼.Since the wavelength band of 13-14 nm is used as the exposurewavelength, an optical path was preliminarily positioned in vacuum.

In the above-mentioned state, the EUV light obtained from the laserplasma X-ray source 32 was incident to the reflective mask 2. The lightreflected by the mask was transferred to the silicon wafer 34 throughthe reducing optical system 33. The light incident to the reflectivemask 2 was absorbed by the absorber (selectively formed on thereflective layer on the substrate 11) and was not reflected in an areawhere the absorber pattern is present. The light incident to an areawhere the absorber pattern is not present was reflected by the EUV lightreflective layer (formed on the substrate 11). Thus, an image formed bythe light reflected from the reflective mask 2 was incident to thereducing optical system 33. The exposure light passing through thereducing optical system 33 exposed a transfer pattern onto a resistlayer on the silicon wafer 34. By developing the resist layer afterexposure, a resist pattern was formed on the silicon wafer 34.

As described above, pattern transfer onto the semiconductor substratewas carried out. As a result, it was confirmed that the reflective maskin this example had an accuracy of 16 nm or less which is a requiredaccuracy in the 70 nm design rule.

From the above-mentioned result, the mask in this example assures a highcontrast for the EUV light and also assures a high contrast for thepattern inspection wavelength. Therefore, pattern inspection can beaccurately and quickly carried out. On the other hand, the existing maskassures a high contrast for the EUV light. However, the contrast for thepattern inspection wavelength is extremely inferior so that accuratepattern inspection is difficult.

Further, a mask was produced in the manner similar to this exampleexcept that a MoSiON thin film was deposited as the inspection lightlow-reflectivity layer 15 in this example. In this case also, a highcontract was obtained for both of the inspection wavelength and the EUVlight, like in this example.

Example 1-2

In the manner similar to Example 1-1, a Mo/Si laminate film ofmolybdenum (Mo) and silicon (Si) was deposited on the substrate 11 asthe EUV light reflective layer 12. On the reflective layer, a SiO₂ thinfilm was deposited to a thickness of 50 nm as the buffer layer 13.

On the buffer layer, a tantalum (Ta) thin film was formed to a thicknessof 50 nm as the exposure light absorbing layer (comprising the EUV lightabsorber) 14. Specifically, using a Ta target, deposition was carriedout by DC magnetron reactive sputtering using an argon gas.

On the exposure light absorbing layer, a TaO thin film was deposited toa thickness of 10 nm as the low-reflectivity layer 15 for the inspectionlight having a wavelength of 257 nm. Specifically, using the same Tatarget and in the same deposition chamber as those in the formation ofthe lower layer of Ta, deposition was carried out by DC magnetronreactive sputtering using a mixed gas of argon and oxygen. The film hada composition of Ta₃₈O₆₂. The TaO film had a refractive index of 2.68and an extinction coefficient of 0.18 for light having a wavelength of260 nm and a refractive index of 2.04 and an extinction coefficient of0.87 for light having a wavelength of 190 nm. The surface of the TaOfilm had a surface roughness of 0.7 nmRms.

In case where the inspection light low-reflectivity layer comprisesoxide of a metal same as that of the EUV light absorbing layer asdescribed in this example, the surface of the EUV light absorbing layermay be treated by an oxidization process using a process gas containingan oxygen gas or an oxidization process using an acid solution.

Using a mask blank produced as mentioned above, an EUV mask having a 16Gbit-DRAM pattern with 0.07 μm design rule was produced in the mannersimilar to Example 1-1.

For the purpose of comparison, preparation was made of a sample of anEUV light absorbing layer as a single layer without the low-reflectivitylayer 15 as the uppermost layer in this example. The EUV light absorbinglayer as a single layer had a thickness of 60 nm equal to the totalthickness of the two layers including the exposure light absorbing layer(EUV light absorbing layer) and the low-reflectivity layer in thisexample.

FIG. 11 shows the values of reflectivity on the surface of the absorberpattern of the mask with respect to the light having a wavelength withina range from 190 nm to 690 nm. In this example, a wavelength regionexhibiting a minimum value of the reflectivity is extremely narrow ascompared with the low-reflectivity layer in Example 1-1.

From the above-mentioned result, in case where the pattern inspectionlight had a wavelength of 257 nm, the reflectivity on the surface of thelow-reflectivity layer of the mask in this example was 4.0% at theabove-mentioned wavelength and the reflectivity of the buffer layer(SiO₂) was 42.1% at the above-mentioned wavelength. Therefore, acontrast between the surface of the low-reflectivity layer and thesurface of the buffer layer at the above-mentioned wavelength was 1:10in terms of the ratio of reflectivity. The contrast value was 83%. Theratio of reflectivity between the surface of the low-reflectivity filmand the surface of the multilayer film was 1:15 and the contrast valuewas 88%.

On the other hand, the reflectivity on the surface of the absorbinglayer of the existing mask was 44% at the above-mentioned wavelength.The contrast between the surface of the absorbing layer and the surfaceof the buffer layer at the above-mentioned wavelength was 1:0.96 interms of the ratio of reflectivity. The contrast value was 2.2%. Theratio of reflectivity between the surface of the absorber and thesurface of the multilayer film was 1:1.4. The contrast value was as lowas 15%.

In the mask in this example, the reflectivity for the EUV light having awavelength of 13.4 nm was 0.5% and 62.4% on the surface of thelow-reflectivity layer as the upper layer of the absorber layer 16 andon the surface of the EUV light reflective layer, respectively. Thecontrast between the surface of the absorber layer 16 and the surface ofthe reflective layer for the EUV light was 1:125 in terms of the ratioof reflectivity. The contrast value was 98%. Similarly, in the existingmask, the contrast between the surface of the single-layer absorbinglayer and the surface of the reflective layer with respect to the EUVlight was 1:105 and the contrast value was 98%.

Using the reflective mask in this example, exposure and transfer werecarried out onto the semiconductor substrate as illustrated in FIG. 9 inthe manner similar to Example 1-1. As a result, it was confirmed thatthe reflective mask had a sufficient exposure characteristic.Specifically, it was confirmed that the EUV reflective mask in thisexample had an accuracy of 16 nm or less which is a required accuracy inthe 70 nm design rule.

From the above-mentioned result, the mask in this example assures a highcontrast for the EUV light and also assures a high contrast for thepattern inspection wavelength. On the other hand, the existing maskassures a high contrast for the EUV light. However, the contrast for thepattern inspection wavelength is extremely inferior.

Example 1-3

In the manner similar to Example 1-1, a Mo/Si laminate film ofmolybdenum (Mo) and silicon (Si) was deposited on the substrate 11 asthe EUV light reflective layer 12. On the reflective layer, a Cr thinfilm was deposited by DC magnetron sputtering to a thickness of 50 nm asthe buffer layer 13. The surface roughness of the surface of the Cr thinfilm was 0.5 nmRms.

In the manner similar to Example 1-2 described above, a tantalum (Ta)thin film was formed on the buffer layer as the exposure light absorbinglayer (comprising the EUV light absorber) 14 and a TaO thin film wasdeposited on the exposure light absorbing layer as the low-reflectivitylayer 15 for the inspection light having a wavelength of 257 nm. In thisexample, the tantalum film had a thickness of 40 nm. The surface of theTaO film had a surface roughness of 0.7 nmRms.

Using a mask blank produced as mentioned above, an EUV reflective maskhaving a 16 Gbit-DRAM pattern with 0.07 μm design rule was produced inthe manner similar to Example 1-1.

For the purpose of comparison, preparation was made of a sample of anEUV light absorbing layer as a single layer without the inspection lightlow-reflectivity layer 15 as the uppermost layer in this example. TheEUV light absorbing layer as a single layer had a thickness of 50 nmequal to the total thickness of the two layer including the exposurelight absorbing layer (EUV light absorbing layer) and the inspectionlight low-reflectivity layer in this example.

FIG. 12 shows the values of reflectivity on the surface of the absorberpattern of the mask with respect to the light having a wavelength withina range from 190 nm to 690 nm.

From the above-mentioned result, in case where the pattern inspectionlight had a wavelength of 257 nm, the reflectivity on the surface of thelow-reflectivity layer of the mask in this example was 4.0% at theabove-mentioned wavelength and the reflectivity of the buffer layer (Cr)was 57.0% at the above-mentioned wavelength. Therefore, a contrastbetween the surface of the low-reflectivity layer and the surface of thebuffer layer at the above-mentioned wavelength was 1:14 in terms of theratio of reflectivity. The contrast value was 87%. The ratio ofreflectivity between the surface of the low-reflectivity film and thesurface of the multilayer film was 1:15 and the contrast value was 88%.

On the other hand, the reflectivity on the surface of the absorbinglayer of the existing mask was 44% at the above-mentioned wavelength.The contrast between the surface of the absorbing layer and the surfaceof the buffer layer at the above-mentioned wavelength was 1:1.3 in termsof the ratio of reflectivity. The contrast value was 13%. The ratio ofreflectivity between the surface of the low-reflectivity layer and thesurface of the multilayer film was 1:1.4. The contrast value was as lowas 15%.

In the mask in this example, the reflectivity for the EUV light having awavelength of 13.4 nm was 0.5% and 62.4% on the surface of thelow-reflectivity layer as the upper layer of the absorber layer 16 andon the surface of the EUV light reflective layer, respectively. Thecontrast between the surface of the absorber layer 16 and the surface ofthe reflective layer for the EUV light was 1:125 in terms of the ratioof reflectivity. The contrast value was 98%. Similarly, in the existingmask, the contrast between the surface of the single-layer absorbinglayer and the surface of the reflective layer with respect to the EUVlight was 1:105 and the contrast value was 98%.

Using the reflective mask in this example, exposure and transfer werecarried out onto the semiconductor substrate as illustrated in FIG. 9 inthe manner similar to Example 1-1. As a result, it was confirmed thatthe reflective mask had a sufficient exposure characteristic.Specifically, it was confirmed that the EUV reflective mask in thisexample had an accuracy of 16 nm or less which is a required accuracy ofthe 70 nm design rule.

From the above-mentioned result, the mask in this example assures a highcontrast for the EUV light and also assures a high contrast for thepattern inspection wavelength. Further, the mask in this example usesthe Cr film as the buffer layer so that the buffer layer also has afunction as the EUV light absorbing layer. It is therefore possible tofurther reduce the thickness of the exposure light absorbing layer (EUVlight absorbing layer) as the upper layer without degrading thecontrast. On the other hand, the existing mask assures a high contrastfor the EUV light. However, the contrast for the pattern inspectionwavelength is extremely inferior.

Example 1-4

In the manner similar to Example 1-1, the EUV light reflective layer 12was deposited on the substrate 11. On the reflective layer 12, achromium nitride film was deposited to a thickness of 50 nm as thebuffer layer 13. The chromium nitride film was formed by DC magnetronsputtering. For deposition, a Cr target was used and a gas containing Arwith 10% nitrogen added thereto was used as a sputter gas.

The chromium nitride film thus deposited had a composition ofCr_(1-x)N_(x) where x=0.1. The chromium nitride film had a film stressof +40 MPa per 100 nm. The chromium nitride film had a reflectivity of52% for the light having a wavelength of 257 nm. The surface roughnessof the surface of the CrN film was 0.27 nmRms.

Next, on the buffer layer 13 comprising the chromium nitride film, atantalum boron nitride (TaBN) film was formed to a thickness of 50 nm asthe exposure light absorbing layer (comprising the EUV light absorber).The TaBN film was formed by DC magnetron sputtering using a targetcontaining Ta and B and a gas containing Ar with 10% nitrogen addedthereto. The TaBN film had a composition ratio of 0.8 Ta, 0.1 B, and 0.1N. The TaBN film had an amorphous structure.

On the TaBN absorbing layer, a tantalum boron nitride (TaBN) film wasformed to a thickness of 15 nm as the low-reflectivity layer 15. TheTaBN film as the low-reflectivity layer was deposited by DC magnetronsputtering using a target containing Ta and B and a gas containing Arwith 40% nitrogen added thereto. At this time, deposition wassuccessively carried out in the same deposition chamber as the TaBN filmas the lower layer, using the same target, changing the amount of thenitrogen gas between the formation of the lower layer and the formationof the upper layer. The TaBN film deposited herein as thelow-reflectivity layer had a composition ratio of 0.5 Ta, 0.1 B, and 0.4N where the ratio of nitrogen is increased as compared with the TaBNfilm as the exposure light absorbing layer (EUV light absorbing layer).The TaBN film was amorphous also.

The TaBN film has a refractive index of 2.3 and an extinctioncoefficient of 1.0 for the light having a wavelength of 257 nm. Theabsorption coefficient for the EUV light having a wavelength of 13.4 nmis 0.036. The surface roughness is 0.25 nmRms. Thus, the film wasextremely smooth.

The reflectivity on the low-reflectivity layer thus obtained was 18% forthe light having a wavelength of 257 nm. The total stress of theexposure light absorbing layer (EUV light absorbing layer) and thelow-reflectivity layer was −50 MPa per 100 nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank produced as mentioned above, an EUVreflective mask having a 16 Gbit-DRAM pattern with 0.07 μm design rulewas produced in the manner similar to Example 1-1.

In the manner similar to Example 1-1, an absorber pattern was at firstformed on the low-reflectivity layer and the exposure light absorbinglayer (EUV light absorbing layer). Herein, using the light having awavelength of 257 nm as inspection light, the absorber pattern wasinspected. The ratio of reflectivity between the buffer layer and thelow-reflectivity layer for the inspection light was 1:0.35. The contrastvalue was 48%. In the inspection, a sufficient contrast was obtained.

Next, the buffer layer comprising chromium nitride was removed by dryetching in conformity with the absorber pattern. The dry etching wascarried out by the use of a mixed gas of chlorine and oxygen.

As described above, a reflective mask in this example was obtained. Thereflective mask thus obtained was again inspected to confirm theabsorber pattern by the use of the inspection light having a wavelengthof 257 nm. As a result, the ratio of reflectivity between the EUVreflective layer and the low-reflectivity layer for the inspection lightwas 1:0.3. The contrast value was 50%. Thus, a sufficient contrast wasobtained In the inspection for confirmation also. For the reflectivemask thus obtained, the reflectivity was measured by the use of the EUVlight having a wavelength of 13.4 nm and an incident angle of 5 degrees.As a result, the reflectivity was 65% and the reflection characteristicwas excellent.

Using the reflective mask in this example, exposure and transfer werecarried out onto the semiconductor substrate as illustrated in FIG. 9 inthe manner similar to Example 1-1. As a result, it was confirmed thatthe reflective mask had a sufficient exposure characteristic.Specifically, it was confirmed that the EUV reflective mask in thisexample had an accuracy of 16 nm or less which is a required accuracy inthe 70 nm design rule.

Example 1-5

This example is different from Example 1-4 in that a tantalum boronoxynitride (TaBNO) film was used as the low-reflectivity layer. In themanner similar to Example 1-4, the EUV light reflective layer 12, thebuffer layer 13, and the exposure light absorbing layer (comprising theEUV light absorber) 14 were deposited on the substrate 11.

Next, on the exposure light absorbing layer (comprising the EUV lightabsorber) 14, the tantalum boron oxynitride (TaBNO) film was formed to athickness of 15 nm as the low-reflectivity layer 15. The TaBNO film wasdeposited by DC magnetron sputtering using a target containing Ta and Band a gas containing Ar with 10% nitrogen and 20% oxygen added thereto.The TaBNO film deposited herein as the low-reflectivity layer had acomposition ratio of 0.4 Ta, 0.1 B, 0.1 N, and 0.4 O. The TaBNOlow-reflectivity layer had a surface roughness of 0.25 nmRms and wasvery smooth. The TaBNO film had an amorphous structure.

The TaBNO film has a refractive index of 2.4 and an extinctioncoefficient of 0.5 for the light having a wavelength of 257 nm. Theabsorption coefficient for the EUV light having a wavelength of 13.4 nmis 0.036. The TaBN layer as the lower layer and the TaBNO layer as theupper layer were successively deposited in the same deposition chamber,changing the species of gas, using the same target.

The reflectivity on the low-reflectivity layer thus obtained was 10% forthe light having a wavelength of 257 nm. The total stress of the EUVlight absorbing layer and the low-reflectivity layer was −50 MPa per 100nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank produced as mentioned above, an EUVreflective mask having a 16 Gbit-DRAM pattern with 0.07 μm design rulewas produced in the manner similar to Example 1-1.

In the manner similar to Example 1-1, an absorber pattern was at firstformed on the low-reflectivity layer and the exposure light absorbinglayer (EUV light absorbing layer). Herein, using the light having awavelength of 257 nm as inspection light, the absorber pattern wasinspected. The ratio of reflectivity between the buffer layer and thelow-reflectivity layer for the inspection light was 1:0.19. The contrastvalue was 68%. In the inspection, a sufficient contrast was obtained.

Next, the buffer layer comprising chromium nitride was removed by dryetching in conformity with the absorber pattern in the manner similar toExample 1-4.

As described above, a reflective mask in this example was obtained. Thereflective mask thus obtained was again inspected to confirm theabsorber pattern by the use of the inspection light having a wavelengthof 257 nm. As a result, the ratio of reflectivity between the EUVreflective layer and the low-reflectivity layer for the inspection lightwas 1:0.17. The contrast value was 71%. Thus, a sufficient contrast wasobtained in the inspection for confirmation also. For the reflectivemask thus obtained, the reflectivity was measured by the use of the EUVlight having a wavelength of 13.4 nm and an incident angle of 5 degrees.As a result, the reflectivity was 65% and the reflection characteristicwas excellent.

Using the reflective mask in this example, exposure and transfer werecarried out onto the semiconductor substrate as illustrated in FIG. 9 inthe manner similar to Example 1-1. As a result, it was confirmed thatthe reflective mask had a sufficient exposure characteristic.Specifically, it was confirmed that the EUV reflective mask in thisexample had an accuracy of 16 nm or less which is a required accuracy inthe 70 nm design rule.

Example 1-6

This example is different from Example 1-4 in that a tantalum boronoxide (TaBO) film was used as the low-reflectivity layer. In the mannersimilar to Example 1-4, the EUV light reflective layer 12, the bufferlayer 13, and the exposure light absorbing layer 14 were deposited onthe substrate 11.

Next, on the exposure light absorbing layer 14, the tantalum boron oxide(TaBO) film was formed to a thickness of 12 nm as the low-reflectivitylayer 15. The TaBO film was deposited by DC magnetron sputtering using atarget containing Ta and B and a gas containing Ar with 30% oxygen addedthereto. Between the formation of the exposure light absorbing layer(EUV light absorbing layer) and the formation of the low-reflectivitylayer, a DC power was stopped and the gas used in deposition waschanged. The TaBO film deposited herein as the low-reflectivity layerhad a composition ratio of 0.4 Ta, 0.1 B, and 0.50. The TaBO film had anamorphous structure.

The TaBO film has a refractive index of 2.5 and an extinctioncoefficient of 0.3 for the light having a wavelength of 257 nm. Theabsorption coefficient for the EUV light having a wavelength of 13.4 nmis 0.035. The TaBO film had a surface roughness of 0.25 nmRms and wasvery smooth.

The reflectivity on the low-reflectivity layer thus obtained was 5% forthe light having a wavelength of 257 nm. The total stress of theexposure light absorbing layer (EUV light absorbing layer) and thelow-reflectivity layer was −50 MPa per 100 nm. The lower layer and theupper layer of the absorber layer were successively deposited in thesame deposition chamber, using the same target, changing the gas.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank produced as mentioned above, an EUVreflective mask having a 16 Gbit-DRAM pattern with 0.07 μm design rulewas produced in the manner similar to Example 1-1.

In the manner similar to Example 1-1, an absorber pattern was at firstformed on the low-reflectivity layer and the absorbing layer. Herein,using the light having a wavelength of 257 nm as inspection light, theabsorber pattern was inspected. The ratio of reflectivity between thebuffer layer and the low-reflectivity layer for the inspection light was1:0.10. The contrast value was 82%. In the inspection, a sufficientcontrast was obtained.

Next, the buffer layer comprising chromium nitride was removed by dryetching in conformity with the absorber pattern in the manner similar toExample 1-4.

As described above, a reflective mask in this example was obtained. Thereflective mask thus obtained was again inspected to confirm theabsorber pattern by the use of the inspection light having a wavelengthof 257 nm. As a result, the ratio of reflectivity between the EUVreflective layer and the low-reflectivity layer for the inspection lightwas 1:0.08. The contrast value was 85%. Thus, a sufficient contrast wasobtained In the inspection for confirmation also. For the reflectivemask thus obtained, the reflectivity was measured by the use of the EUVlight having a wavelength of 13.4 nm and an incident angle of 5 degrees.As a result, the reflectivity was 65% and the reflection characteristicwas excellent.

Using the reflective mask in this example, exposure and transfer werecarried out onto the semiconductor substrate as illustrated in FIG. 9 inthe manner similar to Example 1-1. As a result, it was confirmed thatthe reflective mask had a sufficient exposure characteristic.Specifically, it was confirmed that the EUV reflective mask in thisexample had an accuracy of 16 nm or less which is a required accuracy inthe 70 nm design rule.

Example 1-7 MoSiN as the Upper Layer

In the manner similar to Example 1-4, a Mo/Si reflective multilayerfilm, a CrN buffer layer of 50 nm, and an absorber lower layercomprising a TaBN film of 50 nm were formed on a glass substrate.

Next, as the low-reflectivity layer as the upper layer, a filmcomprising Mo, Si, and N (MoSiN) was formed to a thickness of 10 nm.Deposition was carried out by DC magnetron sputtering using a targetcontaining Si and Mo and a gas containing argon and nitrogen. The MoSiNfilm thus obtained had a composition of Mo:Si:N=23:27:50. The film hadan amorphous structure.

The refractive index and the extinction coefficient for the light havinga wavelength of 260 nm are 2.56 and 0.97, respectively. The refractiveindex and the extinction coefficient for the light having a wavelengthof 190 nm are 2.39 and 1.05, respectively.

The surface of the MoSiN film had a surface roughness of 0.25 nmRms andwas very smooth.

The reflectivity on the surface of the MoSiN film was 17% for theinspection light having a wavelength of 257 nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank, a reflective mask having a 16 Gbit-DRAMpattern with 0.07 μm design rule was produced.

In the manner similar to Example 1-1, a resist pattern was at firstformed on the low-reflectivity layer. Subsequently, by dry etching usinga fluorine gas, the MoSiN low-reflectivity layer was patterned along theresist pattern to expose a part of the TaBN film as the absorber lowerlayer.

Next, by dry etching using a chlorine gas, the TaBN film thus exposedwas patterned in conformity with the MoSiN film to expose a part of theCrN buffer layer.

Herein, in the manner similar to Example 1-1, the absorber pattern wasinspected by the use of the inspection light having a wavelength of 257nm.

The ratio of reflectivity between the surface of the absorber layer andthe surface of the buffer layer for the inspection light was 1:3. Thecontrast value was 50%. Thus, a sufficient contrast was obtained.

In the manner similar to Example 1-1, defects were repaired by FIB.Thereafter, the exposed part of the CrN buffer layer was removed by dryetching using chlorine and oxygen to form a pattern in conformity withthe absorber pattern.

As described above, a reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity between the surface of the absorber layer and thesurface of the multilayer reflective film for the inspection light was1:3.5. The contrast value was 56%. Thus, a sufficient contrast wasobtained.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate (silicon wafer) with a resist in themanner similar to Example 1-1. As a result, it was confirmed that thereflective mask in this example had an accuracy of 16 nm or less whichis a required accuracy in the 70 nm design rule.

Example 1-8 MoSiON as the Upper Layer

In the manner similar to Example 1-4, a Mo/Si reflective multilayerfilm, a CrN buffer layer of 50 nm, and an absorber lower layercomprising a TaBN film of 50 nm were formed on a glass substrate.

Next, as the low-reflectivity layer as the upper layer, a filmcomprising Mo, Si, O, and N (MoSiON) was formed to a thickness of 20 nm.Deposition was carried out by DC magnetron sputtering using a targetcontaining Si and Mo and a gas containing argon, nitrogen, and oxygen.The MoSiON film thus obtained had a composition ofMo:Si:O:N=19:19:19:43. The film had an amorphous structure.

The refractive index and the extinction coefficient for the light havinga wavelength of 260 nm are 2.01 and 0.46, respectively. The refractiveindex and the extinction coefficient for the light having a wavelengthof 190 nm are 1.91 and 0.52, respectively.

The surface of the MoSiON film had a surface roughness of 0.25 nmRms andwas very smooth.

The reflectivity on the surface of the MoSiON film was 4.4% for theinspection light having a wavelength of 257 nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank, a reflective mask having a 16 Gbit-DRAMpattern with 0.07 μm design rule was produced.

In the manner similar to Example 1-1, a resist pattern was at firstformed on the low-reflectivity layer. Subsequently, by dry etching usinga fluorine gas, the MoSiON low-reflectivity layer was patterned alongthe resist pattern to expose a part of the TaBN film as the absorberlower layer.

Next, by dry etching using a chlorine gas, the TaBN film thus exposedwas patterned in conformity with the MoSiN film to expose a part of theCrN buffer layer.

Herein, in the manner similar to Example 1-1, the absorber pattern wasinspected by the use of the inspection light having a wavelength of 257nm.

The ratio of reflectivity between the surface of the absorber layer andthe surface of the buffer layer for the inspection light was 1:12. Thecontrast value was 84%. Thus, a sufficient contrast was obtained.

In the manner similar to Example 1-1, defects were repaired by FIB.Thereafter, the exposed part of the CrN buffer layer was removed by dryetching using chlorine and oxygen to form a pattern in conformity withthe absorber pattern.

As described above, a reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity between the surface of the absorber layer and thesurface of the multilayer reflective film for the inspection light was1:14. The contrast value was 86%. Thus, a sufficient contrast wasobtained.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate (silicon wafer) with a resist in themanner similar to Example 1-1. As a result, it was confirmed that thereflective mask in this example had an accuracy of 16 nm or less whichis a required accuracy in the 70 nm design rule.

Example 1-9 CrO as the Upper Layer

In the manner similar to Example 1-4, a Mo/Si reflective multilayerfilm, a CrN buffer layer of 50 nm, and an absorber lower layercomprising a TaBN film of 50 nm were formed on a glass substrate.

Next, as the low-reflectivity layer as the upper layer, a chromium oxidefilm (CrO) was formed to a thickness of 20 nm. Deposition was carriedout by DC magnetron sputtering using a target containing Cr and a gascontaining argon and oxygen. The CrO film thus obtained had acomposition of Cr:O=46:54.

The refractive index and the extinction coefficient for the light havinga wavelength of 260 nm are 2.37 and 0.72, respectively. The refractiveindex and the extinction coefficient for the light having a wavelengthof 190 nm are 1.91 and 1.13, respectively.

The surface of the CrO film had a surface roughness of 0.3 nmRms.

The reflectivity on the surface of the CrO film was 14% for theinspection light having a wavelength of 257 nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank, a reflective mask having a 16 Gbit-DRAMpattern with 0.07 μm design rule was produced.

In the manner similar to Example 1-1, a resist pattern was at firstformed on the low-reflectivity layer. Subsequently, by dry etching usingchlorine and oxygen, the CrO low-reflectivity layer was patterned alongthe resist pattern to expose a part of the TaBN film as the absorberlower layer.

Next, by dry etching using a chlorine gas, the TaBN film thus exposedwas patterned in conformity with the CrO film to expose a part of theCrN buffer layer.

Herein, in the manner similar to Example 1-1, the absorber pattern wasinspected by the use of the inspection light having a wavelength of 257nm.

The ratio of reflectivity between the surface of the absorber layer andthe surface of the buffer layer for the inspection light was 1:3.7. Thecontrast value was 58%. Thus, a sufficient contrast was obtained.

In the manner similar to Example 1-1, defects were repaired by FIB.Thereafter, the exposed part of the CrN buffer layer was removed by dryetching using chlorine and oxygen to form a pattern in conformity withthe absorber pattern.

As described above, a reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity between the surface of the absorber layer and thesurface of the multilayer reflective film for the inspection light was1:4.3. The contrast value was 62%. Thus, a sufficient contrast wasobtained.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate (silicon wafer) with a resist in themanner similar to Example 1-1. As a result, it was confirmed that thereflective mask in this example had an accuracy of 16 nm or less whichis a required accuracy in the 70 nm design rule.

Example 1-10 SiON as the Upper Layer

In the manner similar to Example 1-4, a Mo/Si reflective multilayerfilm, a CrN buffer layer of 50 nm, and an absorber lower layercomprising a TaBN film of 50 nm were formed on a glass substrate.

Next, as the low-reflectivity layer as the upper layer, a filmcomprising Si, O, and N (SiON) was formed to a thickness of 22 nm.Deposition was carried out by DC magnetron sputtering using a Si targetand a gas containing argon, oxygen, and nitrogen. The SiON film thusobtained had a composition of Si:O:N=28:62:10.

The refractive index and the extinction coefficient for the light havinga wavelength of 260 nm are 1.74 and 0.0018, respectively. The refractiveindex and the extinction coefficient for the light having a wavelengthof 190 nm are 1.86 and 0.0465, respectively.

The surface of the SiON film had a surface roughness of 0.3 nmRms.

The reflectivity on the surface of the SiON film was 5% for theinspection light having a wavelength of 257 nm.

As described above, a reflective mask blank in this example wasobtained.

Next, using the mask blank, a reflective mask having a 16 Gbit-DRAMpattern with 0.07 μm design rule was produced.

In the manner similar to Example 1-1, a resist pattern was at firstformed on the low-reflectivity layer. Subsequently, by dry etching usingfluoride, the SiON low-reflectivity layer was patterned along the resistpattern to expose a part of the TaBN film as the absorber lower layer.

Next, by dry etching using a chlorine gas, the TaBN film thus exposedwas patterned in conformity with the SiON film to expose a part of theCrN buffer layer.

Herein, in the manner similar to Example 1-1, the absorber pattern wasinspected by the use of the inspection light having a wavelength of 257nm.

The ratio of reflectivity between the surface of the absorber layer andthe surface of the buffer layer for the inspection light was 1:10.4. Thecontrast value was 82%. Thus, a sufficient contrast was obtained.

In the manner similar to Example 1-1, defects were repaired by FIB.Thereafter, the exposed part of the CrN buffer layer was removed by dryetching using chlorine and oxygen to form a pattern in conformity withthe absorber pattern.

As described above, a reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity between the surface of the absorber layer and thesurface of the multilayer reflective film for the inspection light was1:12. The contrast value was 85%. Thus, a sufficient contrast wasobtained.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate (silicon wafer) with a resist in themanner similar to Example 1-1. As a result, it was confirmed that thereflective mask in this example had an accuracy of 16 nm or less whichis a required accuracy in the 70 nm design rule.

Example 1-11 Having the TaBN/TaBO Intermediate Region

In the manner similar to Example 1-4, an EUV light reflective layercomprising a Mo/Si periodic laminate film and a buffer layer comprisinga chromium nitride film were formed on a substrate.

Next, as the lower layer of the absorber layer, a tantalum boron nitride(TaBN) film was formed.

The TaBN film was formed by DC magnetron sputtering using a targetcontaining Ta and B and a gas containing Ar with 10% nitrogen addedthereto.

When the TaBN film was formed to about 50 nm, supply of the gascontaining Ar and nitrogen was gradually reduced in ten seconds to bestopped while DC is kept supplied. Simultaneously, oxygen is added to Arup to 30% in the above-mentioned ten seconds without exhausting anddeposition using the same target was continuously carried out in thesame deposition chamber. After the introduction of oxygen is started,about 15 nm deposition was carried out.

The surface of the absorber layer thus formed had a roughness of 0.25nmRms and was very smooth.

The absorber layer had an amorphous structure.

By X-ray photoelectron spectroscopy (XPS), the composition of nitrogenand oxygen in the thickness direction of the absorber layer wasanalyzed. As a result, as illustrated in FIG. 13, it was confirmed thatan intermediate region where the composition was continuously changedfrom the composition of the lower layer towards the composition of theupper layer is formed between the upper and the lower layers. Theintermediate region had a thickness of about 5 nm. In the intermediateregion, the composition is continuously changed in the manner such thatthe content of nitrogen is gradually reduced and the content of oxygenis increased from a lower layer side adjacent to the buffer layertowards an upper layer side adjacent to the surface of the absorberlayer. The lower layer adjacent to the buffer layer was a TaBN filmhaving a composition of Ta:B:N=0.5:0.1:0.4. The upper layer adjacent tothe surface of the absorber layer was a TaBO film having a compositionof Ta:B:O=0.4:0.1:0.5.

The reflectivity on the surface of the absorber layer for the inspectionlight having a wavelength of 257 nm was 5%.

The refractive index and the extinction coefficient of the TaBO film asthe upper layer are 2.5 and 0.3 for the light having a wavelength of 257nm, respectively.

As described above, a reflective mask blank in this example wasobtained.

Next, using the reflective mask blank, a reflective mask having a 16Gbit-DRAM pattern with 0.07 μm design rule was produced.

In the manner similar to Example 1-1, a resist pattern was at firstformed on the low-reflectivity layer. Subsequently, by dry etching usinga gas containing chlorine, the absorber layer was patterned along theresist pattern. The upper layer of the absorber layer, the intermediateregion, and the lower layer were continuously patterned by dry etchingto expose a part of the CrN buffer layer.

Since the intermediate region having continuous change in compositionwas formed between the upper layer and the lower layer, the absorberlayer could be patterned to have an excellent rectangular section whichis continuous without a step. Herein, in the manner similar to Example1-1, the absorber pattern was inspected by the use of the inspectionlight having a wavelength of 257 nm.

The ratio of reflectivity between the surface of the absorber layer andthe surface of the buffer layer for the inspection light was 1:10.4. Thecontrast value was 82%. Thus, a sufficient contrast was obtained.

In the manner similar to Example 1-1, defects were repaired by FIB.Thereafter, the exposed part of the CrN buffer layer was removed by dryetching using chlorine and oxygen to form a pattern in conformity withthe absorber pattern.

As described above, a reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity between the surface of the absorber layer and thesurface of the multilayer reflective film for the inspection light was1:12. The contrast value was 85%. Thus, a sufficient contrast wasobtained.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate (silicon wafer) with a resist in themanner similar to Example 1-1. As a result, it was confirmed that thereflective mask in this example had an accuracy of 16 nm or less whichis a required accuracy in the 70 nm design rule.

The first embodiment of this invention described in the foregoing issummarized as follows.

(1-1) On a substrate, a reflective layer for reflecting exposure lightin a short-wavelength region including an EUV region, a buffer layer forprotecting the reflective layer during formation of a mask pattern, andan absorber layer for absorbing the exposure light are successivelyformed. The absorber layer has an at least two-layer structure includingas a lower layer an exposure light absorbing layer for absorbing theexposure light in the short-wavelength region including the EUV regionand as an upper layer a low-reflectivity layer for inspection light usedin inspection of the mask pattern. With this structure, the absorberlayer on the surface of a reflective mask to be formed is functionallyseparated into a layer for absorbing the exposure light and a layerhaving a low reflectivity for the wavelength of mask pattern inspectionlight. Thus, a sufficient exposure light absorption function is assuredand the reflectivity on the surface of an absorber pattern at thewavelength of the inspection light is remarkably lowered. As a result, adifference in reflectivity between the surface of the absorber patternas an uppermost layer and the surface of the buffer layer in an areawithout the absorber pattern is increased so that a sufficient contrastduring pattern inspection is assured. Therefore, it is possible toaccurately and quickly inspect the mask pattern by the use of anexisting mask inspection apparatus using inspection light in a deepultraviolet region.

(1-2) By selecting a specific substance as an exposure light absorber ofthe lower layer of the absorber layer, the effect of the invention in(1-1) is further exhibited.

(1-3) By selecting a particular substance as the inspection lightlow-reflectivity layer as the upper layer of the absorber layer, theeffect of the invention in (1-1) is further exhibited.

(1-4) A reflective mask obtained by using the mask blank of thisinvention and forming a mask pattern thereon exhibits theabove-mentioned effect.

(1-5) The mask blank of this invention is obtained by the steps offorming on a substrate a reflective layer for reflecting exposure lightin a short-wavelength region including an EUV region, forming on thereflective layer a buffer layer for protecting the reflective layerduring formation of a mask pattern, forming on the buffer layer anexposure light absorbing layer for absorbing the exposure light in theshort-wavelength region including the EUV region, and forming on theexposure light absorbing layer a low-reflectivity layer for inspectionlight used in inspection of the mask pattern. Therefore, a knowndeposition method is applicable. It is thus possible to provide the maskblank easy in production and low in cost.

(1-6) Depending upon a material of an absorber, it is possible to formthe low-reflectivity layer for the inspection light used in inspectionof the mask pattern by forming on the buffer layer the exposure lightabsorbing layer for absorbing the exposure light in the short-wavelengthregion including the EUV region and thereafter treating the surface ofthe absorbing layer. Therefore, it is possible to shorten a timerequired for changing a deposition condition or the like, to reduce thenumber of kinds of materials, and to reduce the number of depositionchambers. Thus, the work is simplified and the working time isshortened.

(1-7) By obtaining the relationship between the thickness of thelow-reflectivity layer formed on the exposure light absorbing layer andthe reflectivity on the low-reflectivity layer at the wavelength of theinspection light, it is possible to control the thickness of thelow-reflectivity layer so as to minimize the reflectivity on thelow-reflectivity layer for the wavelength of the inspection light.

(1-8) The reflective mask of this invention is produced by patterningthe absorber layer in the mask blank. The reflective mask is easilyproduced by the use of known patterning means. It is therefore possibleto provide the reflective mask at a low cost.

(1-9) After patterning the absorber layer of the mask blank, the bufferlayer is removed in an area where the absorber layer is removed.Consequently, the exposure light reflective layer is exposed in an areawhere the absorber pattern is not present. It is therefore possible toimprove an exposure light reflection characteristic of the reflectivemask.

2nd Embodiment

Next, a second embodiment of this invention will be described in detailwith reference to the drawing.

FIG. 14 shows schematic sectional views illustrating a process ofproducing a reflective mask by the use of a reflective mask blank 60according to the second embodiment of this invention.

The reflective mask blank 60 according to this invention comprises asubstrate 61 on which a multilayer reflective film 62, a buffer layer63, and an absorber layer 64 are successively formed, as illustrated inFIG. 14( a).

At first, description will be made of the respective layers of thereflective mask blank 60 according to this invention.

The absorber layer 64 of the reflective mask blank 60 of this inventionhas a function of absorbing EUV light as exposure light.

As the absorber layer 64 of this invention, use is made of a materialcontaining tantalum, boron, and at least one of oxygen and nitrogen. Bythe use of the material, it is possible to sufficiently lower thereflectivity of the absorber layer 64 for a pattern inspectionwavelength as compared with the reflectivity of the buffer layer 63 andto improve a contrast during pattern inspection. Specifically, it isdesired to select a material so that the reflectivity of the surface ofthe absorber layer 64 for the pattern inspection wavelength is 20% orless, preferably 10% or less.

As the above-mentioned material, use may be made of, for example,tantalum boron nitride (TaBN), tantalum boron oxide (TaBO), and tantalumboron oxynitride (TaBNO).

Tantalum is an excellent material of the absorber layer, which has alarge absorption coefficient for the EUV light and which is easilydry-etched by chlorine and excellent in processability.

The tantalum boron alloy (TaB) is easily amorphized and provides a filmexcellent in smoothness. The TaB film is suppressed from reduction ofthe EUV light absorption coefficient as compared with a Ta metal.Further, because of excellent controllability of a film stress, thisfilm is an absorber layer material capable of achieving a highdimensional accuracy of a mask pattern.

By further adding nitrogen to TaB as the above-mentioned material, it ispossible to lower the reflectivity for deep ultraviolet (hereinafterreferred to as DUV) light of 190 nm to 260 nm used as the patterninspection wavelength. By addition of nitrogen, it is possible to obtainan effect of improving the smoothness of the film and reducing thesurface roughness.

If the surface roughness of the surface of the absorber layer is large,edge roughness of the absorber pattern is increased and the dimensionalaccuracy of the pattern is degraded. As the pattern is finer, theinfluence of the edge roughness is remarkable and, therefore, thesurface of the absorber layer is required to be smooth.

In the reflective mask and the reflective mask blank according to thisinvention, the surface roughness of the surface of the absorber ispreferably 0.5 nmRms or less, more preferably 0.4 nmRms or less, furtherpreferably 0.3 nmRms or less.

In order to reduce the surface roughness on the surface of the absorber,it is effective that the absorber layer comprises a film having anamorphous structure. In case where the buffer layer is provided, it isnecessary to use a smooth film as the buffer layer.

By further adding oxygen to TaB, the reflectivity for the DUV light usedas the pattern inspection wavelength is lowered, like in the case ofnitrogen. As compared with nitrogen, oxygen is greater in effect ofreducing the reflectivity for the DUV light. By including both nitrogenand oxygen in TaB, it is possible to obtain the effect of reducing thereflectivity for the pattern inspection wavelength and improving thesmoothness of the film.

Next, a preferable composition ratio will be described with respect totantalum boron nitride (TaBN), tantalum boron oxide (TaBO), and tantalumboron oxynitride (TaBNO) as the material of the absorber layer 4. Inorder that the absorber has a smooth surface, the material is preferablya film having an amorphous structure.

(1) In Case of TaBN

In case of TaBN, the composition of Ta, B, and N is as follows.Preferably, the content of B is 5-25 at %. The ratio of Ta and N (Ta:N)preferably falls within a range of 8:1 to 2:7. The content of B withinthe above-mentioned range is preferable to obtain an amorphous state. Ifthe content of N is small with respect to Ta, a sufficiently lowreflectivity can not be obtained for the inspection light. On thecontrary, if the content of N is large, the film density is lowered. Inthis event, the EUV light absorption coefficient is decreased and theacid resistance is decreased.

(2) In Case of TaBO

In case of TaBO, the content of B is preferably 5-25 at % in order toachieve the amorphous state. The ratio of Ta and O (Ta:O) preferablyfalls within a range of 7:2 to 1:2. If the content of O is small, asufficiently low reflectivity can not be obtained for the inspectionlight. On the other hand, if the content of O is large, the film densityis lowered. The EUV light absorption coefficient is decreased and theinsulation is enhanced to cause easy occurrence of charge-up.

(3) In Case of TaBNO

In case of TaBNO, the content of B is preferably 5-25 at % in order toachieve the amorphous state. The ratio of Ta, N, and O (Ta:(N+O))preferably falls within a range of 7:2 to 2:7. If the content of N and Ois small, a sufficiently low reflectivity can not be obtained for theinspection light. On the contrary, if the content of N and O is large,the film density is lowered. The EUV light absorption coefficient isdecreased and the acid resistance is decreased. Further, the insulationis enhanced to cause each occurrence of charge-up.

As described above, the absorber layer 64 according to this inventionpreferably comprises a material which contains tantalum, boron, and atleast one of oxygen and nitrogen. In addition, an element such as Si,Ge, and Ti may be contained.

The absorber layer 64 according to this invention preferably has anamorphous film structure as described above. A crystalline film issusceptible to stress change with time and is changed in surfacecomposition by an oxygen-containing plasma process. Consequently, thereflectivity for the inspection light is changed. Therefore, in order tobe stable against each of mask cleaning, atmospheric air, and a plasmaenvironment, the absorber layer 64 preferably comprises a film having anamorphous structure without containing a crystalline part.

Preferably, the absorber layer 64 in this invention has an absorptioncoefficient of 0.025 or more, further 0.030 or more at the wavelength ofthe exposure light in order to reduce the thickness of the absorberlayer. The absorber layer 4 must have a thickness sufficient to absorbthe EUV light as the exposure light. Generally, the thickness is on theorder of 30 to 100 nm.

The absorber layer 64 in this invention may be formed by sputtering suchas magnetron sputtering. For example, the absorber layer may bedeposited by sputtering using a target containing tantalum and boron andan argon gas with oxygen or nitrogen added thereto.

The absorber layer 64 in this invention may have a predetermineddistribution of the content of oxygen or nitrogen in the thicknessdirection. In this invention, oxygen or nitrogen may be distributed inthe absorber layer 64 to that the content is increased from the sideadjacent to the buffer layer 3 or the reflective multilayer film towardsthe surface. For example, the content of nitrogen or oxygen distributedin the absorber layer 64 may be increased from the side adjacent to thebuffer layer 3 towards the surface in profile of a linear shape, acurved shape, or a stepwise shape. Such distribution of nitrogen oroxygen may easily be achieved by changing, during deposition, the amountof an oxygen gas or a nitrogen gas added during deposition of theabsorber layer 4.

Alternatively, nitrogen or oxygen may be added only to a predetermineddepth from the surface of the absorber layer 64 (for example, over athickness of about 10 nm to 20 nm from the surface contributing toreflection of the inspection light). Such distribution of nitrogen oroxygen in the absorber layer 64 may be obtained by adjusting the amountof the gas added during deposition as described above. Alternatively,such distribution may be obtained by at first forming the absorber layercontaining Ta and B and then nitriding or oxidizing the surface of theabsorber layer. Such nitriding or oxidizing may be carried out by ionimplantation into the surface of the absorber layer or by exposure ofthe surface of the absorber layer to plasma. Oxidizing may also becarried out by heat treatment.

Generally, as the content of nitrogen or oxygen is increased, theabsorptivity of the EUV light as the exposure light tends to bedecreased. Therefore, a desired distribution is formed in the thicknessof the absorber layer 64 so that the content of nitrogen or oxygen isincreased in the vicinity of the surface of the absorber layer 64contributing to reflection of the inspection light while the content ofnitrogen or oxygen is reduced in a portion adjacent to the buffer layer63 not contributing to reflection of the inspection light, as describedabove. In this event, it is possible to suppress the reduction inabsorptivity of the EUV light in the absorber layer 64 as a whole.

Next, the buffer layer 63 of the reflective mask blank 60 of thisinvention has a function of protecting the multilayer reflective film 2upon forming a pattern on the absorber layer 64 or upon repairing thepattern.

As a material of the buffer layer 63 to be combined with the material ofthe absorber layer 64 of this invention, i.e., the material containingTa, B, and at least one of oxygen and nitrogen, a material containingchromium (Cr) is preferably used.

The buffer layer 63 comprising a material containing Cr assures a largeetch selectivity of 20 or more with respect to the absorber layer 64 ofthis invention containing Ta. The material containing Cr has areflectivity of about 40% to 55% at the pattern inspection wavelengthand is preferable in view of the relationship in reflectivity among thesurface of the multilayer reflective film 62, the surface of the bufferlayer 63, and the surface of the absorber layer 64 (the reflectivity isdesirably smaller in this order) at the inspection wavelength, whichwill later be described. Further, the material containing Cr can beremoved without any substantial damage upon the multilayer reflectivefilm 62 during removal of the buffer layer 63.

As the material containing Cr used as the buffer layer 63 of thisinvention, use is preferably made of a Cr element and a materialcontaining Cr and at least one element selected from N, O, and C. Forexample, use may be made of chromium nitride (CrN), chromium oxide(CrO), chromium carbide (CrC), chromium oxynitride (CrNO), and chromiumcarboxynitride (CrCNO). By adding N to Cr, the acid resistance isimproved so that the resistance against a mask cleaning solution isimproved. In addition, the smoothness of the film is improved and thefilm stress is decreased. By adding O to Cr, low stress controllabilityduring deposition is improved. By adding C to Cr, dry etching resistanceis improved.

For example, in case of chromium nitride (CrN), a preferable compositionratio of chromium and nitrogen is Cr_(1-X)N_(X) where 0.05≦X≦0.5, morepreferably 0.05≦X≦0.2. X being smaller than 0.05 is unfavorable in viewof the acid resistance, the film stress, and the surface roughness. If Xis greater than 0.5, the reflectivity for the inspection light isexcessively lowered so that the contrast with the surface of theabsorber layer 64 can not be sufficiently large. To the Cr_(1-X)N_(X)film, oxygen, carbon, or the like may be added in a small amount, forexample, about 5%.

The buffer layer 63 comprising the material containing Cr may be formedby sputtering such as magnetron sputtering. For example, in case of thechromium nitride film mentioned above, deposition may be carried outusing a Cr target in a gas atmosphere containing argon with 5-40%nitrogen added thereto.

In case where the absorber pattern is repaired by the use of focused ionbeam (hereinafter referred to as FIB), the buffer layer is damaged.Therefore, in order to prevent the multilayer reflective film 2 as theunderlying layer from being affected by the damage, the thickness of thebuffer layer 63 in this invention is preferably 30-50 nm. However, ifthe FIB is not used, the thickness may be as thin as 4-10 nm.

Besides, the material of the buffer layer to be used in combination withthe absorber layer 64 of this invention may be SiO₂, silicon oxynitride(SiON), Ru, and so on.

The buffer layer is formed if necessary. Depending upon the condition ofpattern formation onto the absorber or repair, the absorber layer may beformed directly on the reflective multilayer film.

Next, description will be made of the multilayer reflective film 62 ofthe reflective mask blank 60 according to this invention. As thereflective film 62, a multilayer film comprising elements different inrefractive index and periodically laminated. Generally, use is made of amultilayer film obtained by alternately laminating thin films of a heavyelement or its compound and thin films of a light element or itscompound in about 40 periods.

For example, as a multilayer reflective film for the EUV light having awavelength of 13-14 nm, use is preferably made of a Mo/Si periodicmultilayer film comprising Mo and Si alternately laminated in about 40periods. Besides, as a multilayer reflective film used in the region ofthe EUV light, use may be made of a Ru/Si periodic multilayer film, aMo/Be periodic multilayer film, a Mo compound/Si compound periodicmultilayer film, a Si/Nb periodic multilayer film, a Si/Mo/Ru periodicmultilayer film, a Si/Mo/Ru/Mo periodic multilayer film, and aSi/Ru/Mo/Ru periodic multilayer film. Depending upon the exposurewavelength, a material is appropriately selected.

The multilayer reflective film 62 may be formed by depositing therespective layers by DC magnetron sputtering, ion beam deposition, orthe like. In case of the Mo/Si periodic multilayer film described above,DC magnetron sputtering is used. At first, a Si film of about severalnanometers is deposited using a Si target in an Ar gas atmosphere.Thereafter, a Mo film of about several nanometers is deposited using aMo target in an Ar gas atmosphere. Defining the above-mentioneddeposition as a single period, deposition is carried out in 30 to 60periods. Finally, a Si film is formed.

As the substrate 61 of the reflective mask blank according to thisinvention, it is preferable to use a material having a low thermalexpansion coefficient (within a range of 0±1.0×10⁻⁷/° C., morepreferably within a range of 0±0.3×10⁻⁷/° C.) and excellent insmoothness, flatness, and resistance against a mask cleaning solution.Therefore, use may be made of a glass having a low thermal expansion,for example, a SiO₂—TiO₂ glass. Alternatively, use may be made of acrystallized glass with β-quartz solid solution precipitated therein, aquartz glass, a silicon or a metal substrate, and so on. As the metalsubstrate, an Invar alloy (Fe—Ni alloy) may be used.

Preferably, the substrate 61 has a smooth surface of 0.2 nmRms or lessand a flatness of 100 nm or less in order to achieve high reflectivityand high transfer accuracy. In order to prevent deformation due to afilm stress of a film formed thereon, the substrate 61 preferably hashigh rigidity. In particular, the substrate preferably has a highYoung's modulus of 65 GPa or more.

A unit Rms indicative of the smoothness in this invention is aroot-mean-square roughness and can be measured by the use of an atomicforce microscope. The flatness in this invention is a value indicatingsurface warp (deformation) given by TIR (Total Indicated Reading). Thisvalue is an absolute value of a difference in level between a highestposition on a substrate surface above a focal plane and a lowestposition on the substrate surface below the focal plane where the focalplane is a plane determined by the least square method with reference tothe substrate surface. In this invention, the flatness is measured in anarea of 140 mm×140 mm.

The reflective mask blank 60 according to this invention has theabove-mentioned structure.

Next, description will be made of a production process of the reflectivemask using the reflective mask blank of this invention and patterninspection.

The reflective mask blank 60 (see FIG. 14( a)) in this invention isobtained by successively forming, on the substrate 61, the multilayerreflective film 62, the buffer layer 63, and the absorber layer 64. Thematerial and the forming method of each layer are described above.

Next, an absorber pattern is formed on the absorber layer 64 of thereflective mask blank 60. At first, an electron beam resist is appliedon the absorber layer 64 and baked. Next, writing is carried out by theuse of an electron beam writer and development is carried out to form aresist pattern 65 a.

Using the resist pattern 65 a as a mask, the Ta-based absorber layer 64of this invention is dry-etched using chlorine to form the absorberpattern 64 a (see FIG. 14( b)).

Next, using hot dense sulfuric acid, the resist pattern 65 a remainingon the absorber pattern 64 a is removed to form a mask 66 (see FIG. 14(c)).

Herein, inspection (first inspection) is carried out to confirm whetheror not the absorber pattern 64 a is formed exactly as designed. For theinspection of the absorber pattern 64 a, use is generally made of DUVlight having a wavelength of about 190 nm to 260 nm as described above.The inspection light is incident to the mask 66 with the absorberpattern 64 a formed thereon. Herein, the inspection is carried out bydetecting inspection light reflected on the absorber pattern 64 a andinspection light reflected by the buffer layer 63 exposed after theabsorber layer 64 is removed, and observing the contrast therebetween.

In the above-mentioned manner, detection is made of a pinhole defect(white defect) at which the absorber layer is removed although it shouldnot be removed, or a residue of the absorber layer (black defect) whichis not removed due to underetching. Upon occurrence of the pinholedefect and the underetching defect, these defects are repaired.

The pinhole defect may be repaired by depositing a carbon film using FIBassisted deposition. The underetching defect may be repaired by removingan unnecessary part by FIB irradiation or the like.

After completion of the pattern inspection and the repair as mentionedabove, the exposed part of the buffer layer 63 is removed in conformitywith the absorber pattern 64 a. Thus, a pattern 63 a is formed on thebuffer layer to produce a reflective mask 70 (see FIG. 14( d)). Herein,in case of the buffer layer 63 containing chromium such as chromiumnitride, dry etching with a gas containing chlorine and oxygen may beused.

Finally, the pattern formed as mentioned above is subjected toinspection for final confirmation (final inspection). The inspection forfinal confirmation is to finally confirm whether or not the absorberpattern 64 a has a dimensional accuracy in exact conformity with thespecification. In the inspection for final confirmation also, theabove-mentioned DUV light having a wavelength of about 190 nm to 260 nmis used. The inspection light is incident to the reflective mask 70 inwhich the absorber layer 64 and the buffer layer 63 are patterned. Inthis case, inspection is carried out by detecting inspection lightreflected on the absorber pattern 64 a and inspection light reflected onthe multilayer reflective film 62 exposed after the buffer layer 63 isremoved and observing the contrast therebetween.

Thus, the inspection of the reflective mask includes two kinds, i.e.:

(a) inspection for detecting a pattern defect after forming the absorberpattern (first inspection)

(b) inspection for confirming a final specification of the mask (finalinspection)

In order to accurately and quickly carry out the inspection in each of(a) and (b), it is necessary to obtain a sufficient contrast.

Specifically, the inspection (a) requires the contrast of reflectionbetween the surface of the absorber layer 64 and the surface of thebuffer layer 63. The inspection (b) requires the contrast of reflectionbetween the surface of the absorber layer 64 and the surface of themultilayer reflective film 62.

The contrast value upon inspection is defined by the following formula.contrast value (%)={(R ₂ −R ₁)/(R ₂ +R ₁)}×100

(where R₁ and R₂ are values of reflectivity in the respective layers tobe inspected, R₂>R₁)

The periodic laminate film of Si and Mo generally used as the multilayerreflective film 62 for the EUV light having a wavelength of about 13 nmhas a reflectivity of about 60% for the inspection light (DUV light).Taking the contrast with the multilayer reflective film 62 intoconsideration, it is advantageous to lower the reflectivity on thesurface of the absorber layer 64 for the inspection light. Therefore, inthis invention, it is desired to select the material so that thereflectivity on the surface of the absorber layer 64 is lower than thaton the multilayer reflective film 62.

Further, it is preferable to design the reflectivity so that thereflectivity for the inspection light is successively lowered in theorder of the surface of the multilayer reflective film 2, the surface ofthe buffer layer 63, and the surface of the absorber layer 64. In thismanner, in the inspection in each of (a) and (b) mentioned above, aportion where the absorber pattern 64 a is present is dark withoutreversal of the pattern contrast. Therefore, the setting of theinspection apparatus need not be changed and the result is easilyrecognizable.

In view of the above, it is desired that the surface of the absorberlayer 64 has a reflectivity of 20% or less, preferably 10% or less forthe inspection wavelength. The contrast value (the above-mentioneddefinition formula) in the inspection is 40% or more, preferably 50% ormore, further preferably 60% or more. Herein, the contrast value is acontrast between the absorber layer and the reflective multilayer filmor a contrast between the absorber layer and the buffer layer.

Upon selection of the material of the absorber layer 64 satisfying theabove-mentioned condition, optimization is achieved by preliminarilyobtaining the relationship among the composition of the material of theabsorber layer 64 having the EUV light absorption characteristic, theinspection wavelength, and the reflectivity for the inspection light.For example, with respect to a specific inspection wavelength, therelationship between the composition of the material of the absorberlayer 64 and the reflectivity is obtained. Based on the relationship,the reflectivity on the surface of the absorber layer 4 is adjusted to adesired value. Specifically, the amount of nitrogen or oxygen added toTaB is adjusted so as to achieve a desired reflectivity for thewavelength of light used in the inspection.

The removal of the buffer layer 63 in the mask production processmentioned above may not be carried out if the buffer layer 63 is thinand has less influence upon decrease in reflectivity. In this case, thereflective mask is used in a state where the buffer layer 63 covers anentire surface of the multilayer reflective film 62.

As described above, in this invention, the absorber layer 4 is formed bya material comprising an alloy material containing tantalum and boronexcellent in EUV light absorption and processability and at least one ofnitrogen and oxygen added thereto. It is therefore possible to obtainthe reflective mask blank and the reflective mask having a sufficientcontrast for the inspection light upon inspection of the mask pattern.

Each of the above-mentioned reflective masks and the above-mentionedreflective mask blanks according to this invention is particularlysuitable if the EUV light (having a wavelength of about 0.2-100 nm) isused as the exposure light but may be appropriately used for lighthaving other wavelengths.

Examples

Now, this invention will be described more in detail in connection withspecific examples. For convenience of description, the referencenumerals in FIG. 14 are used as appropriate.

Example 2-1

At first, the reflective mask blank 60 as illustrated in FIG. 14( a) wasproduced. The substrate 61 used herein is a SiO₂—TiO₂ glass substrate(having an outer dimension of 6 inch square and a thickness of 6.3 mm).The substrate 61 has a coefficient of thermal expansion of 0.2×10⁻⁷/° C.and a Young's modulus of 67 GPa. The glass substrate was subjected tomechanical polishing to have a smooth surface of 0.2 nmRms or less and aflatness of 100 nm or less.

As the multilayer reflective film 62 formed on the substrate 61, a Mo/Siperiodic multilayer reflective film was used in this example in order toform the multilayer reflective film suitable for an exposure lightwavelength band of 13-14 nm. Specifically, the multilayer reflectivefilm 62 was formed by alternately laminating Mo and Si on the substrate61 by DC magnetron sputtering. At first, using a Si target, a Si filmwas deposited to the thickness of 4.2 nm under an Ar gas pressure of 0.1Pa. Thereafter, using a Mo target, a Mo film was deposited to athickness of 2.8 nm under an Ar gas pressure of 0.1 Pa. Defining theabove-mentioned deposition as a single period, deposition was carriedout in 40 periods. Finally, a Si film was deposited to a thickness of 4nm. The total thickness was 284 nm. The multilayer reflective film 62had a reflectivity of 65% for light of 13.4 nm at an incident angle of 2degrees. The surface of the multilayer film 62 had a surface roughnessof 0.12 nmRms. The surface of the multilayer reflective film forinspection light having a wavelength of 257 nm was 60%.

The buffer layer 63 formed on the multilayer reflective film 62comprises chromium nitride and has a thickness of 50 nm. Herein,chromium nitride is represented by Cr_(1-X)N_(X) where X=0.1. The bufferlayer 63 was formed by DC magnetron sputtering using a Cr target and asputter gas containing Ar with 10% nitrogen added thereto. By X-raydiffraction, it was confirmed that the buffer layer 63 thus formed had afine crystalline state as a crystal condition.

The buffer layer 63 had a stress of +40 MPa. The surface of the bufferlayer 63 had a reflectivity of 52% for the light having a wavelength of257 nm. The surface of the buffer layer had a surface roughness of 0.27nmRms.

As the absorber layer 64 formed on the buffer layer 63 in this example,tantalum boron nitride (TaBN) was formed to a thickness of 50 nm. Inorder to obtain a desired reflectivity for the inspection light of 257nm, the relationship between the composition of the material of theabsorber layer 64 and the reflectivity for the inspection light of 257nm was obtained and the composition of Ta:B:N was determined as45:10:45. The absorber layer 64 was deposited by DC magnetron sputteringusing a sintered target containing Ta and B and a gas containing Ar with40% nitrogen added thereto. The relationship between a film stress and asupply power to the target was preliminarily obtained. The supply powerto the target was controlled so that the absorber layer 64 had a stressof −50 MPa which is reverse to the stress of the buffer layer 63. Theabsorber layer 64 deposited under the above-mentioned depositioncondition had an amorphous structure. The surface of the absorber layer64 had a reflectivity of 20% for the light of 257 nm and an absorptioncoefficient of 0.036 for the EUV light of a wavelength of 13.4 nm. Thesurface of the absorber layer had a surface roughness of 0.25 nmRms.

As described above, the reflective mask blank 60 in this example wasobtained as illustrated in FIG. 14( a).

Next, description will be made of a method of producing the reflectivemask 70 illustrated in FIG. 14( d). At first, an electron beamirradiation resist was applied on the absorber layer 64 of thereflective mask blank 60. By electron beam, a 16 Gbit-DRAM pattern with0.07 μm design rule was written. Then, development was carried out toform the resist pattern 65 a.

With the resist pattern 65 a used as a mask, the absorber layer 64 wasdry-etched using chlorine to form the absorber pattern 64 a (see FIG.14( b)).

Next, the resist pattern 65 a remaining on the absorber pattern 64 a wasremoved by hot sulfuric acid at 100° C. to obtain the mask 66 (see FIG.14( c)).

In this state, the absorber pattern 64 a was inspected. As illustratedin FIG. 15, the absorber pattern 64 a was inspected by the use of theinspection light having a wavelength of 257 nm and incident to thesurface of the mask 66 and monitoring the contrast between inspectionlight A reflected by the absorber pattern 64 a and inspection light Breflected on the surface of the buffer layer 63.

The ratio of reflectivity for the inspection light between the surfaceof the buffer layer 63 and the surface of the absorber pattern 64 a is1:0.38. The contrast value by the above-mentioned definition formula was44%. Thus, a sufficient contrast was obtained in the inspection.

Next, the chromium nitride layer as the buffer layer 63 remaining on areflection region (an area where the absorber pattern 64 a is absent) ofthe mask 66 was removed in conformity with the absorber pattern 64 a toform a buffer layer pattern 63 a (see FIG. 14( d) described above). Thebuffer layer 63 was removed by dry etching using a mixed gas of chlorineand oxygen.

As described above, the reflective mask 70 having the structureillustrated in FIG. 14( d) was obtained.

After the pattern 63 a was formed on the buffer layer as describedabove, the reflective mask 70 was subjected to the inspection for finalconfirmation. As inspection light, light having a wavelength of 257 nmwas used. The light was incident to the surface of the mask 70 asillustrated in FIG. 16. Observation was made of the contrast betweeninspection light C reflected by the absorber pattern 64 a and inspectionlight D reflected on the multilayer reflective film 62. The ratio ofreflectivity for the inspection light between the surface of themultilayer reflective film 62 exposed after the buffer layer 63 wasremoved and the surface of the absorber pattern 64 a is 1:0.33. Thecontrast value was 50%. Thus, a sufficient contrast was obtained in theinspection for final confirmation also.

By the inspection, it was confirmed that a 16 Gbit-DRAM pattern with0.07 μm design rule was formed on the reflective mask 70 exactly asdesigned.

Next, description will be made of a method of transferring a pattern byEUV light onto a semiconductor substrate (silicon wafer) with a resistby the use of a pattern transfer apparatus shown in FIG. 17 and thereflective mask 70.

The pattern transfer apparatus 50′ with the reflective mask 70 mountedthereto generally comprises a laser plasma X-ray source 32′, thereflective mask 70, and a reducing optical system 33′. The reducingoptical system 33′ comprises an X-ray reflection mirror. The patternreflected by the reflective mask 70 is generally reduced to about ¼ bythe reducing optical system 33′. Since the wavelength band of 13-14 nmis used as the exposure wavelength, an optical path was preliminarilypositioned in vacuum.

In the above-mentioned state, the EUV light obtained from the laserplasma X-ray source 32′ was incident to the reflective mask 70. Thelight reflected by the mask was transferred to the silicon wafer 34′through the reducing optical system 33′.

The light incident to the reflective mask 70 was absorbed by theabsorber layer and was not reflected in an area where the absorberpattern 64 a is present. On the other hand, the light incident to anarea where the absorber pattern 64 a is not present was reflected by themultilayer reflective film 62. Thus, an image formed by the lightreflected from the reflective mask 70 was incident to the reducingoptical system 33′. The exposure light passing through the reducingoptical system 33′ exposed a transfer pattern to a resist layer on thesilicon wafer 34′. By developing the resist layer after exposure, aresist pattern was formed on the silicon wafer 34′.

As described above, pattern transfer onto the semiconductor substratewas carried out. As a result, it was confirmed that the reflective mask70 in this example had an accuracy of 16 nm or less which is a requiredaccuracy in the 70 nm design rule.

Example 2-2

This example is different from Example 2-1 in that tantalum boronoxynitride (TaBNO) was used as a material of the absorber layer 64.

In the manner similar to Example 2-1, the multilayer reflective film 62and the buffer layer 63 of chromium nitride were formed on the substrate61.

Next, on the buffer layer 63, a tantalum boron oxynitride (TaBNO) filmwas formed as the absorber layer 64 to a thickness of 50 nm. Theabsorber layer 64 was formed by DC magnetron sputtering using a targetcontaining Ta and B and a mixed gas containing Ar with 10% nitrogen and20% oxygen added thereto. By controlling the sputtering condition, thestress of the absorber layer 64 was adjusted to −50 MPa.

With reference to the relationship between the composition of TaBNO andthe reflectivity for the inspection light having a wavelength of 257 nm,the composition of the material of the absorber layer 64 was selected sothat a desired reflectivity is obtained. Specifically,Ta:B:N:O=55:10:10:25. The absorber layer 64 thus deposited had anamorphous structure. The surface of the absorber layer 64 had areflectivity of 15% for the light of 257 nm and an absorptioncoefficient of 0.036 for the EUV light having a wavelength of 13.4 nm.The surface roughness on the surface of the absorber layer was 0.25nmRms.

As described above, the reflective mask blank 60 in this example wasobtained.

Next, in the manner similar to Example 2-1, a reflective mask wasproduced from the reflective mask blank in this example.

Specifically, the absorber pattern 64 a was formed on the absorber layer64. After the resist pattern 65 a left on the absorber pattern 64 a wasremoved, the absorber pattern 64 a was inspected in the manner similarto Example 2-1.

In this example, the ratio of reflectivity for the inspection lightbetween the surface of the buffer layer 63 and the surface of theabsorber pattern 64 a was 1:0.29. The contrast value was 55%. Thus, asufficient contrast was obtained in the inspection.

Next, in the manner similar to Example 1-1, the chromium nitride layeras the buffer layer 63 remaining on the reflection region (the areawhere the absorber pattern 64 a is absent) of the mask was removed inconformity with the absorber pattern 64 a to form the buffer layerpattern 63 a. After the pattern 63 a was formed on the buffer layer, thereflective mask 70 was subjected to the inspection for finalconfirmation in the manner similar to Example 2-1.

The ratio of reflectivity for the inspection light between the surfaceof the multilayer reflective film 62 exposed after the buffer layer 63was removed and the surface of the absorber pattern 64 a was 1:0.25. Thecontrast value was 60%. Thus, a sufficient contrast was obtained in theinspection for final confirmation also.

As described above, the reflective mask 70 in this example was obtained.It was confirmed by the inspection that, in the reflective mask 70 inthis example also, a 16 Gbit-DRAM pattern with 0.07 μm design rule wasformed exactly as designed.

Using the reflective mask 70 in this example, pattern transfer onto asilicon wafer was carried out by the use of the pattern transferapparatus illustrated in FIG. 17 in the manner similar to Example 2-1.As a result, it was confirmed that the reflective mask in this examplehad an accuracy of 16 nm or less which is a required accuracy in the 70nm design rule.

Example 2-3

This example is different from Examples 2-1 and 2-2 in that tantalumboron oxide (TaBO) was used as a material of the absorber layer 64.

In the manner similar to Example 2-1, the multilayer reflective film 62and the buffer layer 63 of chromium nitride were formed on the substrate61.

Next, on the buffer layer 63, a tantalum boron oxide (TaBO) film wasformed as the absorber layer 64 to a thickness of 50 nm. The absorberlayer 64 was formed by DC magnetron sputtering using a target containingtantalum and boron and a mixed gas containing Ar with 25% oxygen addedthereto. By controlling the sputtering condition, the stress of theabsorber layer 64 was adjusted to −50 MPa. For the material of theabsorber layer, the relationship between the composition of TaBO and thereflectivity for the inspection light having a wavelength of 257 nm wasobtained. The composition was selected as Ta:B:O=45:10:45 so that adesired reflectivity was obtained. The absorber layer 64 thus depositedhad an amorphous structure. The surface of the absorber layer 64 had areflectivity of 10% for the light of 257 nm and an absorptioncoefficient of 0.035 for the EUV light having a wavelength of 13.4 nm.The surface roughness on the surface of the absorber layer was 0.25nmRms.

As described above, the reflective mask blank 60 in this example wasobtained.

Next, in the manner similar to Example 2-1, a reflective mask wasproduced from the reflective mask blank in this example.

Specifically, the absorber pattern 64 a was formed on the absorber layer64. After the resist pattern 65 a left on the absorber pattern 64 a wasremoved, the absorber pattern 64 a was inspected in the manner similarto Example 1-1.

In this example, the ratio of reflectivity for the inspection lightbetween the surface of the buffer layer 63 and the surface of theabsorber pattern 64 a was 1:0.19. The contrast value was 68%. Thus, asufficient contrast was obtained in the inspection.

Next, in the manner similar to Example 1-1, the chromium nitride layeras the buffer layer 63 remaining in the reflection region (the areawhere the absorber pattern 64 a is absent) of the mask was removed inconformity with the absorber pattern 64 a to form the buffer layerpattern 63 a. After the pattern 63 a was formed on the buffer layer, thereflective mask 70 was subjected to the inspection for finalconfirmation in the manner similar to Example 1-1.

The ratio of reflectivity for the inspection light between the surfaceof the multilayer reflective film 62 exposed after the buffer layer 63was removed and the surface of the absorber pattern 64 a was 1:0.17. Thecontrast value was 71%. Thus, a sufficient contrast was obtained in theinspection for final confirmation also.

As described above, the reflective mask in this example was obtained. Itwas confirmed by the inspection that, in the reflective mask in thisexample, a 16 Gbit-DRAM pattern with 0.07 μm design rule was formedexactly as designed.

Using the reflective mask in this example, pattern transfer onto asilicon wafer was carried out by the use of the pattern transferapparatus illustrated in FIG. 17 in the manner similar to Example 1. Asa result, it was confirmed that the reflective mask in this example hadan accuracy of 16 nm or less which is a required accuracy in the 70 nmdesign rule.

Comparative Example 1

This comparative example is different from Examples 2-1 to 2-3 describedabove in that a tantalum boron alloy (TaB) not containing nitrogen andoxygen was used as a material of the absorber layer 64.

In the manner similar to Example 1-1, the multilayer reflective film 62and the buffer layer 63 of chromium nitride were formed on the substrate61.

Next, on the buffer layer 63, a tantalum boron (TaB) film was formed asthe absorber layer 64 to a thickness of 50 nm. The absorber layer wasformed by DC magnetron sputtering using a target containing tantalum andboron and an Ar gas. By controlling the sputtering condition, the stressof the absorber layer was adjusted to −50 MPa. In the absorber layer,Ta:B=4:1. The absorber layer thus deposited had an amorphous structure.The surface of the absorber layer 64 had a reflectivity of 40% for thelight of 257 nm.

As described above, the reflective mask blank in the comparative examplewas obtained.

Next, in the manner similar to Example 2-1, a reflective mask wasproduced from the reflective mask blank in the comparative example.

At first, the absorber pattern 64 a was formed on the absorber layer 64.After the resist pattern left on the absorber pattern was removed, theabsorber pattern was inspected in the manner similar to Example 1-1.

In the comparative example, the ratio of reflectivity for the inspectionlight between the surface of the buffer layer and the surface of theabsorber pattern was 1:0.77. The contrast value was 13%. Thus, asufficient contrast was not obtained in the inspection.

Next, in the manner similar to Example 2-1, the chromium nitride layeras the buffer layer remaining in the reflection region (the area wherethe absorber pattern is absent) of the mask was removed in conformitywith the absorber pattern 64 a to form the buffer layer pattern. Afterthe pattern was formed on the buffer layer 63, the reflective mask wassubjected to the inspection for final confirmation in the manner similarto Example 2-1.

The ratio of reflectivity for the inspection light between the surfaceof the multilayer reflective film 62 exposed after the buffer layer wasremoved and the surface of the absorber pattern 64 a was 1:0.67. Thecontrast value was 25%. Thus, a sufficient contrast was not obtained inthe inspection for final confirmation.

In the reflective mask in the comparative example, a sufficient contrastwas not obtained as described above. Therefore, an accurate inspectioncould not be carried out. It was therefore impossible to confirm whetheror not a 16 Gbit-DRAM pattern with 0.07 μm design rule was formedexactly as designed.

Example 2-4 Mode without the Buffer Layer, TaBN

In the manner similar to Example 2-1, a Mo/Si periodic multilayerreflective film was formed on a glass substrate. Herein, a Si film as anuppermost layer was formed to a thickness of 11 nm, considering thereduction in film thickness during pattern formation onto the absorberlayer.

The reflectivity on the multilayer reflective film was 60% for theinspection light having a wavelength of 257 nm.

The reflectivity for the EUV light (at an incident angle of 2 degrees)having a wavelength of 13.4 nm was 64%.

Next, as the absorber layer, tantalum boron nitride (TaBN) was formed onthe multilayer reflective film to a thickness of 100 nm.

Considering the reflectivity for the inspection light having awavelength of 257 nm, the composition of the TaBN film was determined asTa:B:N=45:10:45 like in Example.

The TaBN film was formed by DC magnetron sputtering in the mannersimilar to Example 2-1. Herein, as a result of adjusting the supplypower to the target, the TaBN film thus obtained had a stress of −30MPa. The film had an amorphous structure.

The reflectivity on the surface of the TaBN film was 20% for the lightof 257 nm.

The surface roughness was 0.19 nmRms and a very flat surface wasobtained.

As described above, the reflective mask blank in this example wasobtained.

In the manner similar to Example 2-1, a part of the TaBN absorber layerof the reflective mask blank thus obtained was removed and patternedusing a chlorine gas to expose the multilayer reflective film. Thus, theabsorber pattern was formed.

In this state, the absorber pattern was inspected using the inspectionlight having a wavelength of 257 nm. The ratio of reflectivity betweenthe inspection light reflected on the surface of the absorber patternand the inspection light reflected on the surface of the multilayerreflective film was 1:3. The contrast value was 50%. Thus, a sufficientcontrast was obtained.

As described above, pattern inspection of the reflective mask in thisexample was carried out successfully.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate with a resist in the manner similarto Example 1. As a result, it was confirmed that the reflective mask inthis example had an accuracy of 16 nm or less which is a requiredaccuracy in the 70 nm design rule.

Example 2-5 Mode without the Buffer Layer, TaBO

In the manner similar to Example 4, a reflective mask blank and areflective mask were produced except that tantalum boron oxide (TaBO)was used as a material of the absorber layer.

Considering the reflectivity for the inspection light having awavelength of 257 nm, the TaBO absorber layer had a composition ofTa:B:O=45:10:45 similar to Example 3. The thickness was 100 nm.

The TaBO absorber layer was formed by DC magnetron sputtering in themanner similar to Example 3. Herein, as a result of adjusting the supplypower to the target, the TaBO film thus obtained had a stress of −20MPa. The film had an amorphous structure.

The reflectivity on the surface of the TaBO film was 10% for the lightof 257 nm.

The surface roughness was 0.20 nmRms and a very flat surface wasobtained.

As described above, the reflective mask blank in this example wasobtained.

In the manner similar to Example 2-4, a part of the TaBO absorber layerof the reflective mask blank thus obtained was removed and patternedusing a chlorine gas to expose the multilayer reflective film. Thus, theabsorber pattern was formed.

In this state, the absorber pattern was inspected using the inspectionlight having a wavelength of 257 nm. The ratio of reflectivity betweenthe inspection light reflected on the surface of the absorber patternand the inspection light reflected on the surface of the multilayerreflective film was 1:6. The contrast value was 71%. Thus, a sufficientcontrast was obtained.

As described above, pattern inspection of the reflective mask in thisexample was carried out successfully.

Using the reflective mask in this example, pattern transfer was carriedout onto a semiconductor substrate with a resist in the manner similarto Example 1. As a result, it was confirmed that the reflective mask inthis example had an accuracy of 16 nm or less which is a requiredaccuracy in the 70 nm design rule.

Comparative Example 2 SiO₂ Buffer Layer+TaO Single Layer

In the manner similar to Example 2-1, a Mo/Si multilayer reflective filmwas formed on a glass substrate.

Next, on the multilayer reflective film, a SiO₂ film was formed as thebuffer layer to a thickness of 50 nm.

The SiO₂ film was deposited by DC magnetron sputtering using a Si targetand a mixed gas of argon and oxygen.

The surface of the SiO₂ buffer layer had a reflectivity of 42% for theinspection light having a wavelength of 257 nm.

The surface roughness was 0.5 nmRms and was greater than that of the CrNfilm in Examples.

Further, on the SiO₂ buffer layer, the absorber layer of tantalum oxide(TaO) was formed to a thickness of 70 nm.

Formation was carried out by DC magnetron sputtering using a targetcontaining Ta and a mixed gas containing Ar with oxygen added thereto.

The TaO film thus formed had a composition of Ta:O=(60:40) and was acrystalline film.

The surface of the TaO film thus obtained had a reflectivity as low as12% for the inspection light having a wavelength of 257 nm. On the otherhand, the surface roughness was as considerably great as 0.8 nmRms ascompared with Examples of this invention because TaO was the crystallinefilm.

The ratio of reflectivity for the inspect light between the surface ofthe absorber layer and the buffer layer was 1:3.5. The contrast valuewas 56%. The ratio of reflectivity between the surface of the absorberlayer and the surface of the multilayer reflective film was 1:5. Thecontrast value was 67%. Thus, a sufficient contrast was obtained in theinspection.

Using the reflective mask in this comparative example, a pattern wastransferred onto a semiconductor substrate with a resist in the mannersimilar to Example 2-1. As a result, in the reflective mask in thiscomparative example, the edge roughness of the pattern was great becauseof a large surface roughness on the surface of the absorber. Therefore,an accuracy of 16 nm which is a required accuracy in the 70 nm designrule could not be satisfied.

Example 2-6 Mode in which the Composition Varies Towards the Surface(TaBNO)

In the manner similar to Example 2-1, a Mo/Si periodic multilayerreflective film and a CrN buffer layer were formed on a glass substrate.

Next, on the CrN buffer layer, tantalum boron oxide (TaBO) was formed asthe absorber layer to a thickness of 50 nm. The TaBO film was formed byDC magnetron sputtering. A target containing Ta and B was used. A mixedgas of Ar and oxygen was used. Herein, the introduced amount of oxygenwas substantially linearly increased from 0% to 25% with time ofdeposition.

The TaBO film thus obtained had a stress of −50 MPa. The film had anamorphous structure.

Confirmation was made by X-ray photoelectron spectroscopy (XPS). As aresult, the TaBO film thus obtained had a distribution of composition inwhich the content of oxygen is increased in the thickness direction froma side adjacent to the buffer layer towards the surface of the absorber.

The uppermost surface of the TaBO film had a composition ofTa:B:O=45:10:45 approximately.

The reflectivity on the surface of the TaBO film was 12% for the lightof 257 nm.

The surface roughness was 0.24 nmRms and a very smooth surface wasobtained.

As described above, the reflective mask blank in this example wasobtained.

Next, using the mask blank, a reflective mask having a 16 Gbit-DRAMpattern with 0.07 μm design rule was produced.

At first, in the manner similar to Example 2-1, a resist pattern wasformed on the absorber layer. Subsequently, by dry etching using achlorine gas, the TaBO absorber layer was patterned in conformity withthe resist pattern to expose a part of the CrN buffer layer.

Herein, the absorber pattern was inspected using the inspection lighthaving a wavelength of 257 nm.

The ratio of reflectivity for the inspection light between the surfaceof the absorber layer and the surface of the buffer layer was 1:4.3. Thecontrast value was 63%. Thus, a sufficient contrast was obtained.

After detected defects were repaired by FIB, the exposed part of the CrNbuffer layer was removed by dry etching using chlorine and oxygen inconformity with the absorber pattern.

As described above, the reflective mask in this example was obtained.

The reflective mask was subjected to final inspection of the pattern bythe use of the inspection light having a wavelength of 257 nm. The ratioof reflectivity for the inspection light between the surface of theabsorber layer and the surface of the multilayer reflective film was1:5. The contrast value was 67%. Thus, a sufficient contrast wasobtained.

In the manner similar to Example 2-1, pattern transfer was carried outonto a semiconductor substrate (silicon wafer) with a resist by the useof the reflective mask in this example. As a result, it was confirmedthat the reflective mask in this example had an accuracy of 16 nm orless which is a required accuracy in the 0.07 μm design rule.

The second embodiment of this invention described in the foregoing issummarized as follows.

(2-1) In a reflective mask blank comprising a substrate on which amultilayer reflective film for reflecting exposure light, a bufferlayer, and an absorber layer for absorbing the exposure light aresuccessively formed, the absorber layer is made of a material containingtantalum (Ta), boron (B), and nitrogen (N). The composition of Ta, B,and N is selected so that the content of B is 5 at % to 25 at % and theratio of Ta and N (Ta:N) falls within a range of 8:1 to 2:7. Thus, thereflectivity of the absorber layer for the pattern inspection wavelengthis sufficiently lowered so as to improve the contrast during patterninspection. As a result, it is possible to accurately and quickly carryout pattern inspection.

(2-2) The absorber layer is made of a material containing tantalum (Ta),boron (B), and oxygen (O). Like in (2-1), by forming the absorber layerusing a specific material, a sufficient contrast is obtained in patterninspection. It is therefore possible to accurately and quickly carry outpattern inspection.

(2-3) The material of the absorber layer in (2-2) further containsnitrogen (N). Thus, in addition to the effect of (2-2), an effect ofimproving the smoothness of the film of the absorber layer is obtained.

(2-4) The material of the absorber layer has an amorphous state. Thus,in addition to the effects in (2-1) to (2-3), it is possible to obtainan effect of stabilizing the structure of the absorber layer so that thereflectivity for the inspection light is unchanged.

(2-5) The buffer layer is made of a material containing chromium (Cr).Thus, the etch selectivity with the tantalum-based absorber layer inthis invention is large. The relationship with the reflectivity of otherlayers for the inspection wavelength is easily adjustable. Further,there is another effect that the buffer layer can be removed without nosubstantial damage upon the multilayer reflective film.

{2-6) A reflective mask obtained by the use of the reflective mask blankin this invention assures a sufficient contrast in pattern inspectionand enables accurate and quick pattern inspection.

1. A reflective mask blank comprising a substrate on which a reflectivelayer for reflecting exposure light in a short-wavelength regionincluding an extreme ultraviolet region and an absorber layer forabsorbing the exposure light are successively formed, the absorber layerhaving an at least two-layer structure including as a lower layer anexposure light absorbing layer comprising an absorber for the exposurelight and as an upper layer a low-reflectivity layer comprising anabsorber for inspection light used in inspection of a mask pattern, theupper layer being farther from the substrate than the lower layer,wherein the exposure light absorbing layer as the lower layer of theabsorber layer is made of at least one substance selected from a lowerlayer substance group consisting of: one element selected from anelement group consisting of chromium, manganese, cobalt, copper, zinc,gallium, germanium, molybdenum, palladium, silver, cadmium, tin,antimony, tellurium, iodine, hafnium, tantalum, tungsten, titanium, andgold; a substance comprising at least one of nitrogen and oxygen andsaid one element; an alloy comprising said one element; and a substancecomprising at least one of nitrogen and oxygen and said alloy; whereinthe low-reflectivity layer as the upper layer of the absorber layer ismade of at least one substance selected from an upper layer substancegroup consisting of: one of nitride, oxide, and oxynitride of thesubstance forming the exposure light absorbing layer; one of thenitride, the oxide, and the oxynitride with silicon added thereto; andoxynitride of silicon; and wherein a contrast between reflected lightreflected on a surface of the reflective layer and reflected lightreflected on a surface of the absorber layer is 40% or more at thewavelength of light used in inspection of a pattern formed on theabsorber layer.
 2. A reflective mask blank comprising a substrate onwhich a reflective layer for reflecting exposure light in ashort-wavelength region including an extreme ultraviolet region and anabsorber layer for absorbing the exposure light are successively formed,the absorber layer having an at least two-layer structure including as alower layer an exposure light absorbing layer comprising an absorber forthe exposure light and as an upper layer a low-reflectivity layercomprising an absorber for inspection light used in inspection of a maskpattern, the upper layer being farther from the substrate than the lowerlayer, wherein the exposure light absorbing layer as the lower layer ofthe absorber layer is made of at least one substance selected from alower layer substance group consisting of: one element selected from anelement group consisting of chromium, manganese, cobalt, copper, zinc,gallium, germanium, molybdenum, palladium, silver, cadmium, tin,antimony, tellurium, iodine, hafnium, tantalum, tungsten, titanium, andgold; a substance comprising at least one of nitrogen and oxygen andsaid one element; an alloy comprising said one element; and a substancecomprising at least one of nitrogen and oxygen and said alloy; whereinthe low-reflectivity layer as the upper layer of the absorber layer ismade of at least one substance selected from an upper layer substancegroup consisting of: one of nitride, oxide, and oxynitride of thesubstance forming the exposure light absorbing layer; one of thenitride, the oxide, and the oxynitride with silicon added thereto; andoxynitride of silicon; and wherein the reflectivity on the surface ofthe absorber layer is 20% or less at the wavelength of light used ininspection of a pattern formed on the absorber layer.
 3. The reflectivemask blank as claimed in claim 1 or 2, wherein each of the exposurelight absorbing layer and the low-reflectivity layer of the absorberlayer comprises tantalum.
 4. The reflective mask blank as claimed inclaim 1 or 2, wherein the low-reflectivity layer as the upper layer ofthe absorber layer is made of oxide of the substance forming theexposure light absorbing layer.
 5. The reflective mask blank as claimedin claim 1 or 2, wherein the low-reflectivity layer as the upper layerof the absorber layer further comprises boron.
 6. The reflective maskblank as claimed in claim 1 or 2, wherein the exposure light absorbinglayer as the lower layer of the absorber layer is made of a substanceselected from a group consisting of Ta, TaN, TaO, TaSi, TaSiN, TaB,TaBN, TaGe, and TaGeN.
 7. The reflective mask blank as claimed in claim1 or 2, further comprising a buffer layer formed between the reflectivelayer and the absorber layer to protect the reflective layer duringpattern formation on the absorber layer; a contrast between reflectedlight reflected on a surface of the buffer layer and reflected lightreflected on a surface of the absorber layer being 40% or more at thewavelength of light used in inspection of a pattern formed on theabsorber layer.
 8. The reflective mask blank as claimed in claim 7,wherein the buffer layer is formed by Cr or a substance comprising Cr asa main component.
 9. A reflective mask obtained by patterning theabsorber layer of the reflective mask blank claimed in claim 1 or
 2. 10.A method of producing a semiconductor, wherein a pattern is transferredonto a semiconductor substrate by the use of the reflective mask claimedin claim 9.